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  1. 1. INTERNETWORKING BASICSWhat Is an Internet work?An Internet work is a collection of individual networks, connected by intermediate networking devices, thatfunctions as a single large network. Internetworking refers to the industry, products, and procedures that meetthe challenge of creating and administering internet works. Figure 1-1 illustrates some different kinds ofnetwork technologies that can be interconnected by routers and other networking devices to create an internetwork. Figure 1 Different Network Technologies Can Be Connected to Create an Internet work Figure 1History of Internetworking: -The first networks were time-sharing networks that used mainframes and attached terminals. Both IBM’sSystems Network Architecture (SNA) and Digital’s network architecture implemented such environments.Local-area networks (LANs) evolved around the PC revolution. LANs enabled multiple users in a relativelysmall geographical area to exchange files and messages, as well as access shared resources such as file serversand printers.Wide-area networks (WANs) interconnect LANs with geographically dispersed users to create connectivity.Some of the technologies used for connecting LANs include T1, T3, ATM, ISDN, ADSL, Frame Relay, radio links,and others. New methods of connecting dispersed LANs are appearing everyday.Today, high-speed LANs and switched internet works are becoming widely used, largely because they operateat very high speeds and support such high-bandwidth applications as multimedia and videoconferencing.Internetworking evolved as a solution to three key problems: isolated LANs, duplicationof resources, and a lack of network management. Isolated LANs made electronic communication betweendifferent offices or departments impossible. Duplication of resources meant that the same hardware andsoftware had to be supplied to each office or department, as did separate support staff. This lack of networkmanagement meant that no centralized method of managing and troubleshooting networks existed.Internetworking ChallengesThe Technical Zone Page 1
  2. 2. Implementing a functional internetwork is no simple task. Many challenges must be faced, especially in theareas of connectivity, reliability, network management, and flexibility. Each area is key in establishing anefficient and effective internetwork.The challenge when connecting various systems is to support communication among disparate technologies.Different sites, for example, may use different types of media operating at varying speeds, or may even includedifferent types of systems that need to communicate.Because companies rely heavily on data communication, internetworks must provide a certain level ofreliability. This is an unpredictable world; so many large internetworks include redundancy to allow forcommunication even when problems occur.Furthermore, network management must provide centralized support and troubleshooting capabilities in aninternetwork. Configuration, security, performance, and other issues must be adequately addressed for theinternetwork to function smoothly. Security within an internetwork is essential. Many people think of networksecurity from the perspective of protecting the private network from outside attacks. However, it is just asimportant to protect the network from internal attacks, especially because most security breaches come frominside. Networks must also be secured so that the internal network cannot be used as a tool to attack otherexternal sites.Early in the year 2000, many major web sites were the victims of distributed denial of service (DDOS) attacks.These attacks were possible because a great number of private networks currently connected with the Internetwere not properly secured. These private networks were used as tools for the attackers. Because nothing in thisworld is stagnant, internetworks must be flexible enough to change with new demands.Internetworking ModelsWhen networks first came into being, computers could typically communicate only with computers from thesame manufacturer. For example, companies ran either a complete DECnet solution or an IBM solution—notboth together. In the late 1970s, the OSI (Open Systems Interconnection) model was created by the InternationalOrganization for Standardization (ISO) to break this barrier. The OSI model was meant to help vendors createinteroperable network devices. Like world peace, it’ll probably never happen completely, but it’s still a greatgoal. The OSI model is the primary architectural model for networks. It describes how data and networkinformation are communicated from applications on one computer, through the network media, to anapplication on another computer. The OSI reference model breaks this approach into layers.The Layered ApproachA reference model is a conceptual blueprint of how communications should take place. It addresses all theprocesses required for effective communication and divides these processes into logical groupings called layers.When a communication system is designed in this manner, it’s known as layered architecture.Think of it like this: You and some friends want to start a company. One of the first things you’d do is sit downand think through what must be done, who will do them, what order they will be done in, and how they relateto each other. Ultimately, you might group these tasks into departments. Let’s say you decide to have an order-taking department, an inventory department, and a shipping department. Each of your departments has its ownunique tasks, keeping its staff members busy and requiring them to focus on only their own duties.Similarly, software developers can use a reference model to understand computer communication processesand to see what types of functions need to be accomplished on any one layer. If they are developing a protocolfor a certain layer, all they need to concern themselves with is the specific layer’s functions, not those of anyother layer. Another layer and protocol will handle the other functions. The technical term for this idea isbinding. The communication processes that are related to each other are bound, or grouped together, at aparticular layer.The Technical Zone Page 2
  3. 3. Advantages of Reference ModelsThe OSI model is hierarchical, and the same benefits and advantages can apply to any layered model. Theprimary purpose of all models, and especially the OSI model, is to allow different vendors to interoperate. Thebenefits of the OSI model include, but are not limited to, the following: • Dividing the complex network operation into more manageable layers • Changing one layer without having to change all layers. This allows application developers to specialize in design and development. • Defining the standard interface for the “plug-and-play” multi-vendor integrationOpen System Interconnection Reference ModelThe Open System Interconnection (OSI) reference model describes how information from a software applicationin one computer moves through a network medium to a software application in another computer. The OSIreference model is a conceptual model composed of seven layers, each specifying particular network functions.The model was developed by the International Organization for Standardization (ISO) in 1984, and it is nowconsidered the primary architectural model for interceptor communications. The OSI model divides the tasksinvolved with moving information between networked computers into seven smaller, more manageable taskgroups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables thesolutions offered by one layer to be updated without adversely affecting the other layers. The following listdetails the seven layers of the Open System Interconnection (OSI) reference model: • Layer 7—Application • Layer 6—Presentation • Layer 5—Session • Layer 4—Transport • Layer 3—Network • Layer 2—Data link • Layer 1—PhysicalThe OSI Reference Model Contains Seven Independent Layers Application Presentation Sessions Transport Network Data-Link PhysicalCharacteristics of the OSI LayersThe seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers.The upper layers of the OSI model deal with application issues and generally are implemented only in software.The highest layer, the application layer, is closest to the end user. Both users and application layer processesinteract with software applications that contain a communications component. The term upper layer issometimes used to refer to any layer above another layer in the OSI model.The Technical Zone Page 3
  4. 4. The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer areimplemented in hardware and software. The lowest layer, the physical layer, is closest to the physical networkmedium (the network cabling, for example) and is responsible for actually placing information on the medium.Figure 2 illustrates the division between the upper and lower OSI layers.Figure 2: Two Sets of Layers Make Up the OSI Layers Protocols Application Application Presentation Session Transport Network Data Transport Data-Link PhysicalThe OSI model provides a conceptual framework for communication between computers, but the model itself isnot a method of communication. Actual communication is made possible by using communication protocols. Inthe context of data networking, a protocol is a formal set of rules and conventions that governs how computersexchange information over a network medium. A protocol implements the functions of one or more of the OSIlayers.A wide variety of communication protocols exist. Some of these protocols include LAN protocols, WANprotocols, network protocols, and routing protocols. LAN protocols operate at the physical and data link layersof the OSI model and define communication over the various LAN media. WAN protocols operate at the lowestthree layers of the OSI model and define communication over the various wide-area media. Routing protocolsare network layer protocols that are responsible for exchanging information between routers so that therouters can select the proper path for network traffic. Finally, network protocols are the various upper-layerThe Technical Zone Page 4
  5. 5. protocols that exist in a given protocol suite. Many protocols rely on others for operation. For example, manyrouting protocols use network protocols to exchange information between routers. This concept of buildingupon the layers already in existence is the foundation of the OSI model.OSI Model and Communication between SystemsInformation being transferred from a software application in one computer system to a software application inanother must pass through the OSI layers. For example, if a software application in System A has information totransmit to a software application in System B, the application program in System A will pass its information tothe application layer (Layer 7) of System A. The application layer then passes the information to thepresentation layer (Layer 6), which relays the data to the session layer (Layer 5), and so on down to thephysical layer (Layer 1). At the physical layer, the information is placed on the physical network medium and issent across the medium to System B. The physical layer of System B removes the information from the physicalmedium, and then its physical layer passes the information up to the data link layer (Layer 2), which passes itto the network layer (Layer 3), and so on, until it reaches the application layer (Layer 7) of System B. Finally,the application layer of System B passes the information to the recipient application program to complete thecommunication process.Interaction between OSI Model LayersA given layer in the OSI model generally communicates with three other OSI layers: the layer directly above it,the layer directly below it, and its peer layer in other networked computer systems. The data link layer inSystem A, for example, communicates with the network layer of System A, the physical layer of System A, andthe data link layer in System B. Figure 1-4 illustrates this example.Figure 2 OSI Model Layers Communicate with Other LayersFigure 3The Technical Zone Page 5
  6. 6. Figure 3OSI Model Layers and Information ExchangeThe seven OSI layers use various forms of control information to communicate with their peer layers in othercomputer systems. This control information consists of specific requests and instructions that are exchangedbetween peer OSI layers.Control information typically takes one of two forms: headers and trailers. Headers are prepended to data thathas been passed down from upper layers. Trailers are appended to data that has been passed down from upperlayers. An OSI layer is not required to attach a header or a trailer to data from upper layers.Headers, trailers, and data are relative concepts, depending on the layer that analyzes the information unit. Atthe network layer, for example, an information unit consists of a Layer 3 header and data. At the data link layer,however, all the information passed down by the network layer (the Layer 3 header and the data) is treated asdata.In other words, the data portion of an information unit at a given OSI layer potentiallycan contain headers, trailers, and data from all the higher layers. This is known as encapsulation. Figure 1-6shows how the header and data from one layer are encapsulated into the header of the next lowest layer.Figure 4: Headers and Data Can Be Encapsulated During Information Exchange Figure 4The Technical Zone Page 6
  7. 7. Information Exchange ProcessThe information exchange process occurs between peer OSI layers. Each layer in the source system addscontrol information to data, and each layer in the destination system analyzes and removes the controlinformation from that data.If System A has data from a software application to send to System B, the data is passed to the application layer.The application layer in System A then communicates any control information required by the application layerin System B by prepending a header to the data. The resulting information unit (a header and the data) ispassed to the presentation layer, which prepends its own header containing control information intended forthe presentation layer in System B. The information unit grows in size as each layer prepends its own header(and, in some cases, a trailer) that contains control information to be used by its peer layer in System B. At thephysical layer, the entire information unit is placed onto the network medium.The physical layer in System B receives the information unit and passes it to the data link layer. The data linklayer in System B then reads the control information contained in the header prepended by the data link layerin System A. The header is then removed, and the remainder of the information unit is passed to the networklayer. Each layer performs the same actions: The layer reads the header from its peer layer, strips it off, andpasses the remaining information unit to the next highest layer. After the application layer performs theseactions, the data is passed to the recipient software application in System B, in exactly the form in which it wastransmitted by the application in System A.OSI Model Physical LayerThe physical layer defines the electrical, mechanical, procedural, and functional specifications for activating,maintaining, and deactivating the physical link between communicating network systems. Physical layerspecifications define characteristics such as voltage levels, timing of voltage changes, physical data rates,maximum transmission distances, and physical connectors. Physical layer implementations can be categorizedas either LAN or WAN specifications. Figure 1-7 illustrates some common LAN and WAN physical layerimplementations.Figure 5: Physical Layer Implementations Can Be LAN or WAN Specifications Figure 5• OSI Model Data Link LayerThe data link layer provides reliable transit of data across a physical network link. Different data link layerspecifications define different network and protocol characteristics, including physical addressing, networktopology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed tonetwork addressing) defines how devices are addressed at the data link layer. Network topology consists of theThe Technical Zone Page 7
  8. 8. data link layer specifications that often define how devices are to be physically connected, such as in a bus or aring topology. Error notification alerts upper-layer protocols that a transmission error has occurred, and thesequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow controlmoderates the transmission of data so that the receiving device is not overwhelmed with more traffic than itcan handle at one time.The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). Figure 1-8 illustrates the IEEE sub layers ofthe data link layer.Figure 6: The Data Link Layer Contains Two Sub layers Figure 6The Logical Link Control (LLC) sub layer of the data link layer manages communications between devices over asingle link of a network. LLC is defined in the IEEE 802.2 specification and supports both connectionless andconnection-oriented services used by higher-layer protocols. IEEE 802.2 defines a number of fields in data linklayer frames that enable multiple higher-layer protocols to share a single physical data link. The Media AccessControl (MAC) sub layer of the data link layer manages protocol access to the physical network medium. TheIEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one anotherat the data link layer.• OSI Model Network LayerThe network layer defines the network address, which differs from the MAC address. Some network layerimplementations, such as the Internet Protocol (IP), define network addresses in a way that route selection canbe determined systematically by comparing the source network address with the destination network addressand applying the subnet mask. Because this layer defines the logical network layout, routers can use this layerto determine how to forward packets. Because of this, much of the design and configuration work for internetworks happens at Layer 3, the network layer.• OSI Model Transport LayerThe transport layer accepts data from the session layer and segments the data for transport across thenetwork. Generally, the transport layer is responsible for making sure that the data is delivered error-free andin the proper sequence. Flow control generally occurs at the transport layer.Flow control manages data transmission between devices so that the transmitting device does not send moredata than the receiving device can process. Multiplexing enables data from several applications to betransmitted onto a single physical link. Virtual circuits are established, maintained, and terminated by thetransport layer. Error checking involves creating various mechanisms for detecting transmission errors, whileerror recovery involves acting, such as requesting that data be retransmitted, to resolve any errors that occur.The transport protocols used on the Internet are TCP and UDP.Flow Control BasicsFlow control is a function that prevents network congestion by ensuring that transmitting devices do notoverwhelm receiving devices with data. A high-speed computer, for example, may generate traffic faster thanthe network can transfer it, or faster than the destination device can receive and process it. The threeThe Technical Zone Page 8
  9. 9. commonly used methods for handling network congestion are buffering, transmitting source-quench messages,and windowing.Buffering is used by network devices to temporarily store bursts of excess data in memory until they can beprocessed. Occasional data bursts are easily handled by buffering. Excess data bursts can exhaust memory,however, forcing the device to discard any additional datagrams that arrive.Source-quench messages are used by receiving devices to help prevent their buffers from overflowing. Thereceiving device sends source-quench messages to request that the source reduce its current rate of datatransmission. First, the receiving device begins discarding received data due to overflowing buffers. Second, thereceiving device begins sending source-quench messages to the transmitting device at the rate of one messagefor each packet dropped. The source device receives the source-quench messages and lowers the data rate untilit stops receiving the messages. Finally, the source device then gradually increases the data rate as long as nofurther source-quench requests are received.Windowing is a flow-control scheme in which the source device requires an acknowledgment from thedestination after a certain number of packets have been transmitted. With a window size of 3, the sourcerequires an acknowledgment after sending three packets, as follows. First, the source device sends threepackets to the destination device. Then, after receiving the three packets, the destination device sends anacknowledgment to the source. The source receives the acknowledgment and sends three more packets. If thedestination does not receive one or more of the packets for some reason, such as overflowing buffers, it doesnot receive enough packets to send an acknowledgment. The source then retransmits the packets at a reducedtransmission rate.Error-Checking BasicsError-checking schemes determine whether transmitted data has become corrupt or otherwise damaged whiletraveling from the source to the destination. Error checking is implemented at several of the OSI layers.One common error-checking scheme is the cyclic redundancy check (CRC), which detects and discardscorrupted data. Error-correction functions (such as data retransmission) are left to higher-layer protocols. ACRC value is generated by a calculation that is performed at the source device. The destination device comparesthis value to its own calculation to determine whether errors occurred during transmission. First, the sourcedevice performs a predetermined set of calculations over the contents of the packet to be sent. Then, the sourceplaces the calculated value in the packet and sends the packet to the destination. The destination performs thesame predetermined set of calculations over the contents of the packet and then compares its computed valuewith that contained in the packet. If the values are equal, the packet is considered valid. If the values areunequal, the packet contains errors and is discarded.• OSI Model Session LayerThe Session layer is responsible for setting up, managing, and then tearing down sessions between Presentationlayer entities. The Session layer also provides dialog control between devices, or nodes. It coordinatescommunication between systems and serves to organize their communication by offering three differentmodes: • Simplex • half-duplex • full-duplexThe Session layer basically keeps different applications’ data separate from other applications data.• OSI Model Presentation LayerThe Presentation layer gets its name from its purpose: It presents data to the Application layer. It’s essentially atranslator and provides coding and conversion functions. A successful data transfer technique is to adapt thedata into a standard format before transmission. Computers are configured to receive this genericallyformatted data and then convert the data back into its native format for actual reading (for example, EBCDIC toASCII). By providing translation services, the Presentation layer ensures that data transferred from theApplication layer of one system can be read by the Application layer of another host. The OSI has protocolThe Technical Zone Page 9
  10. 10. standards that define how standard data should be formatted. Tasks like data compression, decompression,encryption, and decryption are associated with this layer. Some Presentation layer standards are involved inmultimedia operations. The following serve to direct graphic and visual image presentation:PICT: This is picture format used by Macintosh or PowerPC programs for transferring Quick Draw graphics.TIFF: The Tagged Image File Format is a standard graphics format for high-resolution, bitmapped images.JPEG: The Joint Photographic Experts Group brings these photo standards to us. Other standards guide moviesand sound.MIDI: The Musical Instrument Digital Interface is used for digitized music.MPEG: The Moving Picture Experts Group’s standard for the compression and coding of motion video for CDs isincreasingly popular. It provides digital storage and bit rates up to 1.5Mbps .• OSI Model Application LayerThe application layer is the OSI layer closest to the end user, which means that both the OSI application layerand the user interact directly with the software application.This layer interacts with software applications that implement a communicating component. Such applicationprograms fall outside the scope of the OSI model. Application layer functions typically include identifyingcommunication partners, determining resource availability, and synchronizing communication.When identifying communication partners, the application layer determines the identity and availability ofcommunication partners for an application with data to transmit. When determining resource availability, theapplication layer must decide whether sufficient network resources for the requested communication exist. Insynchronizing communication, all communication between applications requires cooperation that is managedby the application layer.Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and SimpleMail Transfer Protocol (SMTP).Information FormatsThe data and control information that is transmitted through internetworks takes a variety of forms. The termsused to refer to these information formats are not used consistently in the internetworking industry butsometimes are used interchangeably. Common information formats include frames, packets, datagrams,segments, messages, cells, and data units.A frame is an information unit whose source and destination are data link layer entities. A frame is composed ofthe data link layer header (and possibly a trailer) and upper-layer data. The header and trailer contain controlinformation intended for the data link layer entity in the destination system. Data from upper-layer entities isencapsulated in the data link layer header and trailer. Figure 1-9 illustrates the basic components of a data linklayer frame.Figure 7: Data from Upper-Layer Entities Makes Up the Data Link Layer Frame Figure 7A packet is an information unit whose source and destination are network layer entities. A packet is composedof the network layer header (and possibly a trailer) and upper-layer data. The header and trailer containcontrol information intended for the network layer entity in the destination system. Data from upper-layerentities is encapsulated in the network layer header and trailer. Figure 1-10 illustrates the basic components ofa network layer packet.The Technical Zone Page 10
  11. 11. Figure 8: Three Basic Components Make Up a Network Layer Packet Figure 8The term datagram usually refers to an information unit whose source and destination are network layerentities that use connectionless network service.The term segment usually refers to an information unit whose source and destination are transport layerentities.A message is an information unit whose source and destination entities exist above the network layer (often atthe application layer).A cell is an information unit of a fixed size whose source and destination are data link layer entities. Cells areused in switched environments, such as Asynchronous Transfer Mode (ATM) and Switched Multimegabit DataService (SMDS) networks. A cell is composed of the header and payload. The header contains controlinformation intended for the destination data link layer entity and is typically 5 bytes long. The payloadcontains upper-layer data that is encapsulated in the cell header and is typically 48 bytes long.The length of the header and the payload fields always are the same for each cell.Figure 1picts the components of a typical cell.Figure below Two Components Make Up a Typical Cell Figure 9Data unit is a generic term that refers to a variety of information units. Some common data units are servicedata units (SDUs), protocol data units, and bridge protocol data units (BPDUs). SDUs are information units fromupper-layer protocols that define a service request to a lower-layer protocol. PDU is OSI terminology for apacket. BPDUs are used by the spanning-tree algorithm as hello messages .Connection-Oriented and Connectionless Network ServicesIn general, transport protocols can be characterized as being either connection-oriented or connectionless.Connection-oriented services must first establish a connection with the desired service before passing any data.A connectionless service can send the data without any need to establish a connection first. In general,connection-oriented services provide some level of delivery guarantee, whereas connectionless services do not.Connection-oriented service involves three phases: connection establishment, data transfer, andconnection termination.During connection establishment, the end nodes may reserve resources for the connection. The end nodes alsomay negotiate and establish certain criteria for the transfer, such as a window size used in TCP connections.This resource reservation is one of the things exploited in some denial of service (DOS) attacks. An attackingsystem will send many requests for establishing a connection but then will never complete the connection. TheThe Technical Zone Page 11
  12. 12. attacked computer is then left with resources allocated for many never-completed connections. Then, when anend node tries to complete an actual connection, there are not enough resources for the valid connection.The data transfer phase occurs when the actual data is transmitted over the connection. During data transfer,most connection-oriented services will monitor for lost packets and handle resending them. The protocol isgenerally also responsible for putting the packets in the right sequence before passing the data up the protocolstack.When the transfer of data is complete, the end nodes terminate the connection and release resources reservedfor the connection.Connection-oriented network services have more overhead than connectionless ones. Connection-orientedservices must negotiate a connection, transfer data, and tear down the connection, whereas a connectionlesstransfer can simply send the data without the added overhead of creating and tearing down a connection. Eachhas its place in internetworks.MAC AddressesMedia Access Control (MAC) addresses consist of a subset of data link layer addresses. MAC addresses identifynetwork entities in LANs that implement the IEEE MAC addresses of the data link layer. As with most data-linkaddresses, MAC addresses are unique for each LAN interface. Figure 1-14 illustrates the relationship betweenMAC addresses, data-link addresses, and the IEEE sub layers of the data link layer.Figure 10: MAC Addresses, Data-Link Addresses, and the IEEE Sub layers of the Data Link LayerAre All Related Figure 10MAC addresses are 48 bits in length and are expressed as 12 hexadecimal digits. The first 6 hexadecimal digits,which are administered by the IEEE, identify the manufacturer or vendor and thus comprise theOrganizationally Unique Identifier (OUI). The last 6 hexadecimal digits comprise the interface serial number, oranother value administered by the specific vendor. MAC addresses sometimes are called burned-in addresses(BIAs) because they are burned into read-only memory (ROM) and are copied into random-access memory(RAM) when the interface card initializes. Figure 1-15 illustrates the MAC address format.The Technical Zone Page 12
  13. 13. Figure 11: The MAC Address Contains a Unique Format of Hexadecimal Digits Figure 11Mapping AddressesBecause internetworks generally use network addresses to route traffic around the network, there is a need tomap network addresses to MAC addresses. When the network layer has determined the destination stationsnetwork address, it must forward the information over a physical network using a MAC address. Differentprotocol suites use different methods to perform this mapping, but the most popular is Address ResolutionProtocol (ARP). Different protocol suites use different methods for determining the MAC address of a device.The following three methods are used most often. Address Resolution Protocol (ARP) maps network addressesto MAC addresses. The Hello protocol enables network devices to learn the MAC addresses of other networkdevices. MAC addresses either are embedded in the network layer address or are generated by an algorithm.Address Resolution Protocol (ARP) is the method used in the TCP/IP suite. When a network device needs tosend data to another device on the same network, it knows the source and destination network addresses forthe data transfer. It must somehow map the destination address to a MAC address before forwarding the data.First, the sending station will check its ARP table to see if it has already discovered this destination stationsMAC address. If it has not, it will send a broadcast on the network with the destination stations IP addresscontained in the broadcast. Every station on the network receives the broadcast and compares the embeddedIP address to its own. Only the station with the matching IP address replies to the sending station with a packetcontaining the MAC address for the station. The first station then adds this information to its ARP table forfuture reference and proceeds to transfer the data.When the destination device lies on a remote network, one beyond a router, the process is the same except thatthe sending station sends the ARP request for the MAC address of its default gateway. It then forwards theinformation to that device. The default gateway will then forward the information over whatever networksnecessary to deliver the packet to the network on which the destination device resides. The router on thedestination devices network then uses ARP to obtain the MAC of the actual destination device and delivers thepacket. The Hello protocol is a network layer protocol that enables network devices to identify one another andindicate that they are still functional. When a new end system powers up, for example, it broadcasts hellomessages onto the network. Devices on the network then return hello replies, and hello messages are also sentat specific intervals to indicate that they are still functional. Network devices can learn the MAC addresses ofother devices by examining Hello protocol packets..The Technical Zone Page 13
  14. 14. Network Layer AddressesA network layer address identifies an entity at the network layer of the OSI layers. Network addresses usuallyexist within a hierarchical address space and sometimes are called virtual or logical addresses.The relationship between a network address and a device is logical and unfixed; it typically is based either onphysical network characteristics (the device is on a particular network segment) or on groupings that have nophysical basis (the device is part of an AppleTalk zone). End systems require one network layer address foreach network layer protocol that they support. (This assumes that the device has only one physical networkconnection.) Routers and other internetworking devices require one network layer address per physicalnetwork connection for each network layer protocol supported. For example, a router with three interfaceseach running AppleTalk, TCP/IP, and OSI must have three network layer addresses for each interface. Therouter therefore has nine network layer addresses. Figure 1-16 illustrates how each network interface must beassigned a network address for each protocol supported.Figure 12: Each Network Interface Must Be Assigned a Network Address for Each Protocolsupported Figure 12The Technical Zone Page 14
  15. 15. Address AssignmentsAddresses are assigned to devices as one of two types: static and dynamic. Static addresses are assigned by anetwork administrator according to a preconceived internetwork addressing plan. A static address does notchange until the network administrator manually changes it. Dynamic addresses are obtained by devices whenthey attach to a network, by means of some protocol-specific process. A device using a dynamic address oftenhas a different address each time that it connects to the network. Some networks use a server to assignaddresses. Server-assigned addresses are recycled for reuse as devices disconnect. A device is therefore likelyto have a different address each time that it connects to the network.Addresses versus NamesInternet work devices usually have both a name and an address associated with them. Internet work namestypically are location-independent and remain associated with a device wherever that device moves (forexample, from one building to another). Internetwork addresses usually are location-dependent and changewhen a device is moved (although MAC addresses are an exception to this rule). As with network addressesbeing mapped to MAC addresses, names are usually mapped to network addresses through some protocol. TheInternet uses Domain Name System (DNS) to map the name of a device to its IP address. For example, its easierfor you to remember instead of some IP address. Therefore, you type into yourbrowser when you want to access Ciscos web site. Your computer performs a DNS lookup of the IP address forCiscos web server and then communicates with it using the network address.TCP/IP ModelThe TCP/IP model is a condensed version of the OSI model. It is comprised of four, instead of seven, layers: • The Process/Application layer • The Host-to-Host layer • The Internet layer • The Network Access layerFigure given bellow shows a comparison of the TCP/IP or DoD model and the OSI reference model. As you cansee, the two are similar in concept, but each has a different number of layers with different names.A vast array of protocols combines at the DoD model’s Process/Application layer to integrate the variousactivities and duties spanning the focus of the OSI’s corresponding top three layers (Application, Presentation,and Session). The Process/Application layer defines protocols for node-to-node application communication andalso controls user-interface specifications. The Host-to-Host layer parallels the functions of the OSI’s TransportThe Technical Zone Page 15
  16. 16. layer, defining protocols for setting up the level of transmission service for applications. It tackles issues likecreating reliable end-to-end communication and ensuring the error-free delivery of data. It handles packetsequencing and maintains data integrity.The Internet layer corresponds to the OSI’s Network layer, designating the protocols relating to the logicaltransmission of packets over the entire network. It takes care of the addressing of hosts by giving them an IP(Internet Protocol) address, and it handles the routing of packets among multiple networks. It also controls thecommunication flow between two hosts. At the bottom of the model, the Network Access layer monitors thedata exchange between the host and the network. The equivalent of the Data Link and Physical layers of the OSImodel, the Network Access layer oversees hardware addressing and defines protocols for the physicaltransmission of data. While the DoD and OSI models are alike in design and concept and have similar functionsin similar places, how those functions occur is different. Figure given bellow shows the TCP/IP protocol suiteand how its protocols relate to the DoD model layers.The Process/Application Layer ProtocolsIn this section, we will describe the different applications and services typically used in IP networks. Thedifferent protocols and applications covered in this section include the following: • TELNET • FTP • TFTP • NFS • SMTP • LPD • X Window • SNMP • DNS • DHCPTelnetTelnet is the chameleon of protocols—its specialty is terminal emulation. It allows a user on a remote clientmachine, called the Telnet client, to access the resources of another machine, the Telnet server. Telnet achievesthis by pulling a fast one on the Telnet server and making the client machine appear as though it were aterminal directly attached to the local network. This projection is actually a software image, a virtual terminalThe Technical Zone Page 16
  17. 17. that can interact with the chosen remote host. These emulated terminals are of the text-mode type and canexecute refined procedures like displaying menus that give users the opportunity to choose options from themand access the applications on the duped server. Users begin a Telnet session by running the Telnet clientsoftware and then logging on to the Telnet server.File Transfer Protocol (FTP)The File Transfer Protocol (FTP) is the protocol that actually lets us transfer files; it can facilitate this betweenany two machines using it. But FTP isn’t just a protocol; it’s also a program. Operating as a protocol, FTP is usedby applications. As a program, it’s employed by users to perform file tasks by hand. FTP also allows for accessto both directories and files and can accomplish certain types of directory operations, like relocating intodifferent ones. FTP teams up with Telnet to transparently log you in to the FTP server and then provides for thetransfer of files. Accessing a host through FTP is only the first step, though. Users must then be subjected to anauthentication login that’s probably secured with passwords and usernames implemented by systemadministrators to restrict access. But you can get around this somewhat by adopting the username“anonymous”—though what you’ll gain access to will be limited. Even when employed by users manually as aprogram, FTP’s functions are limited to listing and manipulating directories, typing file contents, and copyingfiles between hosts. It can’t execute remote files as programs.Trivial File Transfer Protocol (TFTP)The Trivial File Transfer Protocol (TFTP) is the stripped-down, stock version of FTP, but it’s the protocol ofchoice if you know exactly what you want and where to find it. It doesn’t give you the abundance of functionsthat FTP does, though. TFTP has no directory-browsing abilities; it can do nothing but send and receive files.This compact little protocol also skimps in the data department, sending much smaller blocks of data than FTP,and there’s no authentication as with FTP, so it’s insecure. Few sites support it because of the inherent securityrisks.Network File System (NFS)Network File System (NFS) is a jewel of a protocol specializing in file sharing. It allows two different types offile systems to interoperate. It works like this: Suppose the NFS server software is running on an NT server, andthe NFS client software is running on a Unix host. NFS allows for a portion of the RAM on the NT server totransparently store Unix files, which can, in turn, be used by Unix users. Even though the NT file system andUnix file system are unlike—they have different case sensitivity, filename lengths, security, and so on—bothUnix users and NT users can access that same file with their normal file systems, in their normal way.Simple Mail Transfer Protocol (SMTP)Simple Mail Transfer Protocol (SMTP), answering our ubiquitous call to e-mail, uses a spooled, or queued,method of mail delivery. Once a message has been sent to a destination, the message is spooled to a device—usually a disk. The server software at the destination posts a vigil, regularly checking this queue for messages.When it detects them, it proceeds to deliver them to their destination. SMTP is used to send mail; POP3 is usedto receive mail.Line Printer Daemon (LPD)The Line Printer Daemon (LPD) protocol is designed for printer sharing. The LPD, along with the LPR (LinePrinter) program, allows print jobs to be spooled and sent to the network’s printers using TCP/IP.X WindowDesigned for client-server operations, X Window defines a protocol for the writing of graphical user interface–based client/server applications. The idea is to allow a program, called a client, to run on one computer andhave it display a program called a window server on another computer.Simple Network Management Protocol (SNMP)The Technical Zone Page 17
  18. 18. Simple Network Management Protocol (SNMP) collects and manipulates this valuable network information. Itgathers data by polling the devices on the network from a management station at fixed or random intervals,requiring them to disclose certain information. When all is well, SNMP receives something called a baseline— areport delimiting the operational traits of a healthy network. This protocol can also stand as a watchdog overthe network, quickly notifying managers of any sudden turn of events. These network watchdogs are calledagents, and when aberrations occur, agents send an alert called a trap to the management station.Domain Name Service (DNS)Domain Name Service (DNS) resolves host names, specifically Internet names, like Youdon’t have to use DNS; you can just type in the IP address of any device you want to communicate with. An IPaddress identifies hosts on a network and the Internet as well. However, DNS was designed to make our liveseasier. Also, what would happen if you wanted to move your Web page to a different service provider? The IPaddress would change and no one would know what the new one was. DNS allows you to use a domain name tospecify an IP address. You can change the IP address as often as you want and no one will know the difference.The Host-to-Host Layer ProtocolsThe Host-to-Host layer’s main purpose is to shield the upper-layer applications from the complexities of thenetwork. This layer says to the upper layer, “Just give me your data stream, with any instructions, and I’ll beginthe process of getting your information ready to send.” The following sections describe the two protocols at thislayer: • Transmission Control Protocol (TCP) • User Datagram Protocol (UDP)Transmission Control Protocol (TCP) The Transmission Control Protocol (TCP) takes large blocks of information from an application and breaksthem into segments. It numbers and sequences each segment so that the destination’s TCP protocol can put thesegments back into the order the application intended. After these segments are sent, TCP (on the transmittinghost) waits for an acknowledgment of the receiving end’s TCP virtual circuit session, retransmitting those thataren’t acknowledged. Before a transmitting host starts to send segments down the model, the sender’s TCPprotocol contacts the destination’s TCP protocol to establish a connection. What is created is known as a virtualcircuit. This type of communication is called connection-oriented. During this initial handshake, the two TCPlayers also agree on the amount of information that’s going to be sent before the recipient’s TCP sends back anacknowledgment. With everything agreed upon in advance, the path is paved for reliable communication totake place. TCP is a full-duplex, connection-oriented, reliable, accurate protocol, and establishing all these termsand conditions, in addition to error checking, is no small task. TCP is very complicated and, not surprisingly,costly in terms of network overhead. Since today’s networks are much more reliable than those of yore, thisadded reliability is often unnecessary.User Datagram Protocol (UDP)Application developers can use the User Datagram Protocol (UDP) in place of TCP. UDP is the scaled-downeconomy model and is considered a thin protocol. Like a thin person on a park bench, a thin protocol doesn’ttake up a lot of room—or in this case, much bandwidth on a network. UDP also doesn’t offer all the bells andwhistles of TCP, but it does do a fabulous job of transporting information that doesn’t require reliable delivery— and it does so using far fewer network resources. There are some situations where it would definitely bewise for application developers to opt for UDP rather than TCP. Remember the watchdog SNMP up there at theProcess/Application layer? SNMP monitors the network, sending intermittent messages and a fairly steady flowof status updates and alerts, especially when running on a large network. The cost in overhead to establish,maintain, and close a TCP connection for each one of those little messages would reduce what would be anotherwise healthy, efficient network to a dammed-up bog in no time. Another circumstance calling for UDPover TCP is when the matter of reliability is already accomplished at the Process/Application layer. NetworkFile System (NFS) handles its own reliability issues, making the use of TCP both impractical and redundant.However, the application developer decides whether to use UDP or TCP, not the user who wants to transferThe Technical Zone Page 18
  19. 19. data faster. UDP receives upper-layer blocks of information, instead of data streams as TCP does, and breaksthem into segments. Like TCP, each UDP segment is given a number for reassembly into the intended block atthe destination. However, UDP does not sequence the segments and does not care in which order the segmentsarrive at the destination. At least it numbers them, though. But after that, UDP sends the segments off andforgets about them. It doesn’t follow through, check up on them, or even allow for an acknowledgment of safearrival—complete abandonment. Because of this, it’s referred to as an unreliable protocol. This does not meanthat UDP is ineffective, only that it doesn’t handle issues of reliability. Further, UDP doesn’t create a virtualcircuit, nor does it contact the destination before delivering information to it. It is, therefore, also considered aconnectionless protocol. Since UDP assumes that the application will use its own reliability method, it doesn’tuse any. This gives an application developer a choice when running the Internet Protocol stack: TCP forreliability or UDP for faster transfers.The Internet Layer ProtocolsThere are two main reasons for the Internet layer’s existence: routing, and providing a single network interfaceto the upper layers. None of the upper- or lower-layer protocols have any functions relating to routing. Thecomplex and important task of routing is the job of the Internet layer. The Internet layer’s second job is toprovide a single network interface to the upper-layer protocols. Without this layer, application programmerswould need to write “hooks” into every one of their applications for each different Network Access protocol.This would not only be a pain in the neck, but it would lead to different versions of each application—one forEthernet, another one for Token Ring, and so on. To prevent this, IP provides one single network interface forthe upper-layer protocols. That accomplished, it’s then the job of IP and the various Network Access protocolsto get along and work together. All network roads don’t lead to Rome—they lead to IP. And all the otherprotocols at this layer, as well as all those at the upper layers, use it. Never forget that. All paths through themodel go through IP. The following sections describe the protocols at the Internet layer. These are theprotocols that work at the Internet layer: • Internet Protocol (IP) • Internet Control Message Protocol (ICMP) • Address Resolution Protocol (ARP) • Reverse Address Resolution Protocol (RARP)Internet Protocol (IP)The Internet Protocol (IP) essentially is the Internet layer. The other protocols found here merely exist tosupport it. IP contains the big picture and could be said to “see all,” in that it is aware of all the interconnectednetworks. It can do this because all the machines on the network have software, or logical, address called an IPaddress. IP looks at each packet’s address. Then, using a routing table, it decides where a packet is to be sentnext, choosing the best path. The Network Access–layer protocols at the bottom of the model don’t possess IP’senlightened scope of the entire network; they deal only with physical links (local networks). Identifying deviceson networks requires answering these two questions: Which network is it on? And what is its ID on thatnetwork? The first answer is the software, or logical, address (the correct street). The second answer is thehardware address (the correct mailbox). All hosts on a network have a logical ID called an IP address. This isthe software, or logical, address and contains valuable encoded information greatly simplifying the complextask of routing. IP receives segments from the Host-to-Host layer and fragments them into datagrams (packets).IP then reassembles datagrams back into segments on the receiving side. Each datagram is assigned the IPaddress of the sender and of the recipient. Each router (layer-3 device) that receives a datagram makes routingdecisions based upon the packet’s destination IP address. IP protocol has to go through every time user data issent from the upper layers and wants to be sent to a remote network.Internet Control Message Protocol (ICMP)The Internet Control Message Protocol (ICMP) works at the Network layer and is used by IP for many differentservices. ICMP is a management protocol and messaging service provider for IP. Its messages are carried as IPdatagrams. RFC 1256, ICMP Router Discovery Messages, is an annex to ICMP, which affords hosts’ extendedThe Technical Zone Page 19
  20. 20. capability in discovering routes to gateways. Periodically, router advertisements are announced over thenetwork, reporting IP addresses for the routers network interfaces. Hosts listen for these network infomercialsto acquire route information. A router solicitation is a request for immediate advertisements and may be sentby a host when it starts up. If a router can’t send an IP datagram any further, it uses ICMP to send a messageback to the sender, advising it of the situation. For example, if a router receives a packet destined for a networkthat the router doesn’t know about, it will send an ICMP Destination Unreachable message back to the sendingstation.Buffer Full: If a router’s memory buffer for receiving incoming datagrams is full, it will use ICMP to send outthis message.Hops: Each IP datagram is allotted a certain number of routers, called hops, which it may go through. If itreaches its limit of hops before arriving at its destination, the last router to receive that datagram deletes it. Theexecutioner router then uses ICMP to send an obituary message, informing the sending machine of the demiseof its datagram.Ping: Packet Internet Groper uses ICMP echo messages to check the physical connectivity of machines on aninternetwork.Trace route: Using ICMP timeouts, trace route is used to find a path a packet takes as it traverses aninternetwork. The following data is from a network analyzer catching an ICMP echo request. Notice that eventhough ICMP works at the Network layer, it still uses IP to do the Ping request.Address Resolution Protocol (ARP)The Address Resolution Protocol (ARP) finds the hardware address of a host from a known IP address. Here’show it works: When IP has a datagram to send, it must inform a Network Access protocol, such as Ethernet orToken Ring, of the destination’s hardware address on the local network. (It has already been informed byupper-layer protocols of the destination’s IP address.) If IP doesn’t find the destination host’s hardware addressin the ARP cache, it uses ARP to find this information. As IP’s detective, ARP interrogates the local network bysending out a broadcast asking the machine with the specified IP address to reply with its hardware address. Inother words, ARP translates the software (IP) address into a hardware address—for example, the destinationmachine’s Ethernet board address—and from it, deduces its whereabouts. This hardware address is technicallyreferred to as the media access control (MAC) address or physical address. Figure given bellow shows how anARP might look to a local network.Reverse Address Resolution Protocol (RARP)When an IP machine happens to be a diskless machine, it has no way of initially knowing its IP address, but itdoes know its MAC address. The Reverse Address Resolution Protocol (RARP) discovers the identity of the IPaddress for diskless machines by sending out a packet that includes its MAC address and a request for the IPThe Technical Zone Page 20
  21. 21. address assigned to that MAC address. A designated machine, called a RARP server, responds with the answer,and the identity crisis is over. RARP uses the information it does know about the machine’s MAC address tolearn its IP address and complete the machine’s ID portrait.Ways of CommunicationUnicasting • Communication between two devices is one-on-one. Create least traffic while communicating. Best in when one device want to communicate with one device only as no extra bothering the other hosts on the segment. Cannot be use in one-on-many devices to communicate as one hub device need to send the many copies of the same packet to all the hosts and will get the Acks from them.Broadcasting • Communication between two devices is one-on-all. One-n-all means all the host in the network on the same switch. When host send the packet on broadcast address then the switch will duplicate the packet and will send it on all the host in the network.Multicasting • Communication with one-on-one and one-on-many has too many limitations like large traffic to handle and security breach. It is used when one-on-group one way communication is required. For example live telecasting of video stream on internet, in this case the users are group of people who may need the particular stream but not all the hosts. So the user will join the particular multicast group to get that particular stream.IP AddressingOne of the most important topics in any discussion of TCP/IP is IP addressing. An IP address is anumeric identifier assigned to each machine on an IP network. It designates the location of a device onthe network. An IP address is a software address, not a hardware address—the latter is hardcoded ona network interface card (NIC) and used for finding hosts on a local network. IP addressing wasdesigned to allow a host on one network to communicate with a host on a different network,regardless of the type of LANs the hosts is participating in.IP stands for Internet Protocol, its a communications protocol used from the smallest private networkto the massive global Internet. An IP address is a unique identifier given to a single device on an IPnetwork. The IP address consists of a 32-bit number that ranges from 0 to 4294967295. This meansthat theoretically, the Internet can contain approximately 4.3 billion unique objects. But to make suchThe Technical Zone Page 21
  22. 22. a large address block easier to handle, it was chopped up into four 8-bit numbers, or "octets,"separated by a period. Instead of 32 binary base-2 digits, which would be too long to read, itsconverted to four base-256 digits. Octets are made up of numbers ranging from 0 to 255. The numbersbelow show how IP addresses increment. 252 hosts... 252 hosts.. 4+ billion hosts... TerminologyHere are a few of the most important terms: -Bit One digit; either a 1 or a 0.Byte 8 bits.Octet Always 8 bits. Base-8 addressing scheme.Network address The designation used in routing to send packets to a remote network, for example,,, and address Used by applications and hosts to send information to all nodes on a network. Examples include255.255.255.255, which is all networks, all nodes;, which is all subnets and hosts on network17.16.0.0; and, which broadcasts to all subnets and hosts on network Technical Zone Page 22
  23. 23. The Hierarchical IP Addressing SchemeAn IP address consists of 32 bits of information. These bits are divided into four sections, referred to as octetsor bytes, each containing 1 byte (8 bits).You can depict an IP address using one of three methods: • Dotted-decimal, as in • Binary, as in 10101100.00010000.00011110.00111000 • Hexadecimal, as in 82 39 1E 38Network AddressingThe Technical Zone Page 23
  24. 24. The network address uniquely identifies each network. Every machine on the same network shares thatnetwork address as part of its IP address. In the IP address, for example, 172.16 is the networkaddress.The node address is assigned to, and uniquely identifies, each machine on a network. This part of the addressmust be unique because it identifies a particular machine—an individual—as opposed to a network, which is agroup. This number can also be referred to as a host address. In the sample IP address, .30.56 isthe node address. The designers of the Internet decided to create classes of networks based on network size.For the small number of networks possessing a very large number of nodes, they created the rank Class Anetwork. At the other extreme is the Class C network, which is reserved for the numerous networks with a smallnumber of nodes. The class distinction for networks between very large and very small is predictably called theClass B network. Subdividing an IP address into a network and node address is determined by the classdesignation of one’s network. Figure summarizes the three classes of networks: -Network Address Range: Class AThe designers of the IP address scheme said that the first bit of the first byte in a Class A network address mustalways be off, or 0. This means a Class A address must be between 0 and 127.Here is how those numbers are defined:0xxxxxxx: If we turn the other 7 bits all off and then turn them all on, we will find your Class A range of networkaddresses.00000000=001111111=127Network Address Range: Class BIn a Class B network, the RFCs state that the first bit of the first byte must always be turned on, but the secondbit must always be turned off. If you turn the other six bits all off and then all on, you will find the range for aClass B network:10000000=12810111111=191As you can see, this means that a Class B network can be defined when the first byte is configured from 128 to191.Network Address Range: Class CFor Class C networks, the RFCs define the first two bits of the first octet always turned on, but the third bit cannever be on. Following the same process as the previous classes, convert from binary to decimal to find therange.Here is the range for a Class C network:11000000=19211011111=223The Technical Zone Page 24
  25. 25. So, if you see an IP address that starts at 192 and goes to 223, you’ll know it is a Class C IP address.Network Address Ranges: Classes D and EThe addresses between 224 and 255 are reserved for Class D and E networks.Class D is used for multicast addresses and Class E for scientific purposes.Network Addresses: Special PurposeSome IP addresses are reserved for special purposes, and network administrators shouldn’t assign theseaddresses to nodes. Table given bellow lists the members of this exclusive little club and why they’re includedin it.Network –Id • Can be defined as the Id to represent the no. of host addresses in the same network in the topology. Cannot be assign to any host in the network. When all the host past is zero then it is called network-id. Or simply the first address of the network is always Network-IdBroadcast-Id • Address on which if packets are send these will be receive by all the hosts in the network. T his address is used when all the host in the network are suppose to get the same message. Cannot be assign to any host in the network. When all the host bits are one then it is called broadcast-id. Simply the last address of the network is called broadcast-id.Class A AddressesIn a Class A network address, the first byte is assigned to the network address and the three remaining bytesare used for the node addresses. The Class A format is Network.Node.Node.Node For example, in the IPaddress, 49 is the network address, and 22.102.70 is the node address. Every machine on thisparticular network would have the distinctive network address of 49. Class A addresses are one byte long, withThe Technical Zone Page 25
  26. 26. the first bit of that byte reserved and the seven remaining bits available for manipulation. As a result, themaximum number of Class A networks that can be created is 128. Why?Because each of the seven bit positions can either be a 0 or a 1, thus 27 or 128.To complicate matters further, the network address of all 0s (0000 0000) is reserved to designate the defaultroute. Additionally, the address 127, which is reserved for diagnostics, can’t be used either, which means thatyou can only use the numbers 1 to 126 to designate Class A network addresses. This means the actual numberof usable Class A network addresses is 128 minus 2, or 126. Got it? Each Class A address has three bytes (24-bitpositions) for the node address of a machine. Thus, there are 224—or 16,777,216—unique combinations and,therefore, precisely that many possible unique node addresses for each Class A network. Because addresseswith the two patterns of all 0s and all 1s are reserved, the actual maximum usable number of nodes for a ClassA network is 224 minus 2, which equals 16,777,214.Class A Valid Host IDsHere is an example of how to figure out the valid host IDs in a Class A network address: All host bits offis the network address. All host bits on is the broadcast address. The valid hosts are thenumber in between the network address and the broadcast address: through Noticethat 0s and 255s are valid host IDs. All you need to remember when trying to find valid host addresses is thatthe host bits cannot all be turned off or on at the same time.Class B AddressesIn a Class B network address, the first two bytes are assigned to the network address, and the remaining twobytes are used for node addresses. The format is Network. Network. Node. Node. For example, in the IPaddress, the network address is 172.16, and the node address is 30.56. With a network addressbeing two bytes (eight bits each), there would be 216 unique combinations. But the Internet designers decidedthat all Class B network addresses should start with the binary digit 1, then 0. This leaves 14 bit positions tomanipulate, therefore 16,384 (214) unique Class B network addresses. A Class B address uses two bytes fornode addresses. This is 216 minus thetwo reserved patterns (all 0s and all 1s), for a total of 65,534 possiblenode addresses for each Class B network.Class B Valid Host IDsHere is an example of how to find the valid hosts in a Class B network: All host bits turned off is thenetwork address. All host bits turned on is the broadcast address. The valid hosts would be thenumbers in between the network address and the broadcast address: through C AddressesThe first three bytes of a Class C network address are dedicated to the network portion of the address, withonly one measly byte remaining for the node address. The format is Network.Network.Network.Node. Usingthe example IP address, the network address is192.168.100, and the node address is 102.In aClass C network address, the first three bit positions are always the binary 110. The calculation is such: 3 bytes,or 24 bits, minus 3 reserved positions, leaves 21 positions. Hence, there are 221, or 2,097,152, possible Class Cnetworks. Each unique Class C network has one byte to use for node addresses. This leads to 28 or 256,minus the two reserved patterns of all 0s and all 1s, for a total of 254 node addresses for each Class C network.Class C Valid Host IDsHere is an example of how to find a valid host ID in a Class C network: All host bits turned off isthe network ID. All host bits turned on is the broadcast address. The valid hosts would be thenumbers in between the network address and the broadcast address: through while assigning IP addresses to host, two addresses can never assign one Network-Id and other isBroadcast-Id. Always subtract 2 from the total no of IPs in the network. Network Subnet-mask Total No. of Usable Network –Id IPs IPs Broadcast-IdThe Technical Zone Page 26
  27. 27. 2^24 2^24 - 2 / 65536 65534 / 256 254 / word subnet is short for sub network--a smaller network within a larger one. The smallest subnetthat has no more subdivisions within it is considered a single "broadcast domain," which directlycorrelates to a single LAN (local area network) segment on an Ethernet switch. The broadcast domainserves an important function because this is where devices on a network communicate directly witheach others MAC addresses, which dont route across multiple subnets, let alone the entire Internet.MAC address communications are limited to a smaller network because they rely on ARP broadcastingto find their way around, and broadcasting can be scaled only so much before the amount of broadcasttraffic brings down the entire network with sheer broadcast noise. For this reason, the most commonsmallest subnet is 8 bits, or precisely a single octet, although it can be smaller or slightly larger.Subnetting is just the concept of borrowing the bits from the host part to reduce the host part and toinclude it in the network part. With this the no. of available network will be increase and the no ofhosts the subnetted will be decreased. This way more efficient assignment of IP addressing in thenetwork is possible with least possible wasting of IPs as they very limited in no .in IPv4Subnets have a beginning and an ending, and the beginning number is always even and the endingnumber is always odd. The beginning number is the "Network ID" and the ending number is the"Broadcast ID." Youre not allowed to use these numbers because they both have special meaning withspecial purposes. The Network ID is the official designation for a particular subnet, and the endingnumber is the broadcast address that every device on a subnet listens to.With the Subnetting one bigger network can break down into smaller no. of Sub networks. With eachsub network they must have their own Network-Id and Broadcast-Id.For example192.168.1.0 Broadcast-Id doing binary of last octet we will get following192.168.0.00000000Now here we have last 8 digits as host bits and first 24 bits are for network and are reserve.Lets we have N no. of requirement of IP addressesNow we have to find out how many bits are suppose to require to reserve for hosts and rest left bitsare subnet bitsWith N no. of hosts we require one Network-Id and Broadcast-Id so total no. of IPs required areN + 2. To generate N options we need M(say) bits to reserve for network. N + 2 ≤ 2^M (General for all classes)Now the No. of Subnet Networks will be as given below 2^ (8-M)Considering the requirement of 60 peopleThe Technical Zone Page 27
  28. 28. No. of Ips required are N + 2 = 62 where N = 60By putting the values we will get M = 6So no of Subnets will be 2^(8-6) = 4And no. of people in the each subnet will be is 2^6 = 64192.168.0. 00 000000 Subnet bits Host bitsNow Ist will be192.168.0.00 ****** Decimal Form ****** Decimal Form ****** Decimal Form ****** Decimal Form Broadcast-Id Network-Id Broadcast- Id Decimal Form192.16 8.0.00000000 192.168.127192.168.0.10000000 Variable Length Subnet Masking (VLSM)Conventional Subnet masking replaces the two-level IP addressing scheme with a more flexible three-level method. Since it lets network administrators assign IP addresses to hosts based on how they areconnected in physical networks, subnetting is a real breakthrough for those maintaining large IPnetworks. It has its own weaknesses though, and still has room for improvement. The main weaknessof conventional subnetting is in fact that the subnet ID represents only one additional hierarchicallevel in how IP addresses are interpreted and used for routing.The Problem With Single-Level SubnettingIt may seem “greedy” to look at subnetting and say “what, only one additional level”? J However, inlarge networks, the need to divide our entire network into only one level of subnetworks doesntrepresent the best use of our IP address block. Furthermore, we have already seen that since thesubnet ID is the same length throughout the network, we can have problems if we have subnetworkswith very different numbers of hosts on them—the subnet ID must be chosen based on whicheversubnet has the greatest number of hosts, even if most of subnets have far fewer. This is inefficienteven in small networks, and can result in the need to use extra addressing blocks while wasting manyof the addresses in each block.For example, consider a relatively small company with a Class C network, They havesix subnetworks in their network. The first four subnets (S1, S2, S3 and S4) are relatively small,containing only 10 hosts each. However, one of them (S5) is for their production floor and has 50hosts, and the last (S6) is their development and engineering group, which has 100 hosts.The total number of hosts needed is thus 196. Without subnetting, we have enough hosts in our ClassC network to handle them all. However, when we try to subnet, we have a big problem. In order tohave six subnets we need to use 3 bits for the subnet ID. This leaves only 5 bits for the host ID, whichmeans every subnet has the identical capacity of 30 hosts. This is enough for the smaller subnets butThe Technical Zone Page 28
  29. 29. not enough for the larger ones. The only solution with conventional subnetting, other than shufflingthe physical subnets, is to get another Class C block for the two big subnets and use the original for thefour small ones. But this is expensive, and means wasting hundreds of IP addresses.Suppose requirement is as following.120 People for marketing people60 people for Finance30 Tell callers14 Team Leaders6 Managers2 Directors2 Senate MembersThe Technical Zone Page 29
  30. 30. TRANSMISSION MEDIUM USEDUnshielded Twisted Pair (UTP) CableUnshielded Twisted Pair (UTP) is undoubtedly the most common transmission system. Twisted paircables are available unshielded (UTP) or shielded (STP). UTP is the most common. STP is used in noisyenvironments where the shield protects against excessive electromagnetic interference. Both UTP andSTP come in stranded and solid wire varieties. The stranded wire is the most common and is also veryflexible for bending around corners. Solid wire cable has less attenuation and can span longerdistances, but is less flexible than stranded wire and cannot be repeatedly bent Shielded Twisted Pair (STP) involves a metal foil, or shield, that surrounds each pair in a cable,sometimes with another shield surrounding all the pairs in a multi-pair cable.The shields serve to block ambient interference by absorbing it and conducting it to ground. Thatmeans that the foils have to be spliced just as carefully as the conductors, and that the connection toground has to be rock-solid.Twisted pair comes in following categories: - 1. UTP Analog voice 2. UTP Digital voice (1 Mbps data) 3. UTP, STP Digital voice (16 Mbps data) 4. UTP, STP Digital voice (20 Mbps data) 5. UTP, STP Digital voice (100 Mbps data)Unshielded Twisted Pair (UTP) CableTwisted pair cabling comes in two varieties: shielded and unshielded . Unshielded twisted pairThe quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The cable has four pairsof wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminateinterference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supportedtransmission rate and the greater the cost per foot.Unshielded Twisted Pair ConnectorThe Technical Zone Page 30
  31. 31. The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector thatlooks like a large telephone-style connector (fig.). A slot allows the RJ-45 to be inserted only one way. RJ standsfor Registered Jack, implying that the connector follows a standard borrowed from the telephone industry. Thisstandard designates which wire goes with each pin inside the connector.The RJ-45 connector is clear so you can see the eight colored wires that connect to the connector’s pins. Thesewires are twisted into four pairs. Four wires (two pairs) carry the voltage and are considered tip. The other fourwires are grounded and are called ring. The RJ-45 connector is crimped onto the end of the wire, and thepin locations of the connector are numbered from the left, 8 to 1. RJ-45 connectorPin Wire Pair (T is tip, R is Ring) 1 Pair 2 T2 2 Pair 2 R2 3 Pair 3 T3 4 Pair 1 R1 5 Pair 1 T1 6 Pair 3 R3 7 Pair 4 T4 8 Pair 4 R4Straight-ThroughIn a UTP implementation of a straight-through cable, the wires on both cable ends are in the same order.You can use a straight-through cable for the following tasks:  Connecting a router to a hub or switch  Connecting a server to a hub or switch  Connecting workstations to a hub or switchCrossoverIn the implementation of a crossover, the wires on each end of the cable are crossed. Transmit toreceive and receive to Transmit on each side, for both tip and ring.You can use a crossover cable for the following tasks:  Connecting uplinks between switches  Connecting hubs to switches  Connecting a hub to another hubCoaxial CableCoaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the centerconductor and a braided metal shield. The metal shield helps to block any outside interference from fluorescentlights, motors, and other computers.The Technical Zone Page 31
  32. 32. Coaxial cableAlthough coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it cansupport greater cable lengths between network devices than twisted pair cable. The two types of coaxial cablingare thick coaxial and thin coaxial.Coaxial Cable ConnectorsThe most common type of connector used with coaxial cables is the Bayone-Neill-Concelman (BNC) connector.Different types of adapters are available for BNC connectors, including a T-connector, barrel connector, andterminator. Connectors on the cable are the weakest points in any network. BNC connectorFiber Optic CableFiber optic cabling consists of a center glass core surrounded by several layers of protective materials. Ittransmits light rather than electronic signals eliminating the problem of electrical interference. This makes itideal for certain environments that contain a large amount of electrical interference. It has also made it thestandard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pair. Italso has the capability to carry information at vastly greater speeds. This capacity broadens communicationpossibilities to include services such as video conferencing and interactive services. Fiber optic cableFiber Optic ConnectorThe most common connector used with fiber optic cable is an ST connector. It is barrel shaped, similar to a BNCconnector. A newer connector, the SC has a squared face and is easier to connect in a confined space .SwitchesSwitch is an intelligent device that forwards only those packets that are meant for that subnet.Here we will discuss in detail 3com super stack 3300 switch in detail: -3com Switch:The Super Stack 3 Switch 3300 connects your existing 10Mbps devices, connects high-performanceworkgroups with a 100Mbps backbone or server connection, and connects power users to dedicated100Mbps ports - all in one switch. In addition, as part of the 3Com Super Stack 3 range of products,you can combine it with any Super Stack 3 system as your network grows.The Technical Zone Page 32
  33. 33. Features:The Switch has the following hardware features:• 12 or 24 Fast Ethernet auto-negotiating 10BASE-T/100BASE-TX ports• Matrix port for connecting units in the Switch 1100/3300 family to form a stack:• Connect two units back-to-back using a single Matrix Cable• Connect up to four units using Matrix Cables linked to a Matrix Module• Slot for an Expansion ModuleFront view:Rear View:Switches occupy the same place in the network as hubs. Unlike hubs, switches examine each packet and processit accordingly rather than simply repeating the signal to all ports. Switches map the Ethernet addresses of thenodes residing on each network segment and then allow only the necessary traffic to pass through the switch.When a packet is received by the switch, the switch examines the destination and source hardware addresses andcompares them to a table of network segments and addresses. If the segments are the same, the packet isdropped ("filtered"); if the segments are different, then the packet is "forwarded" to the proper segment.Additionally, switches prevent bad or misaligned packets from spreading by not forwarding them.The Technical Zone Page 33
  34. 34. Filtering of packets and the regeneration of forwarded packets enables switching technology to split a networkinto separate collision domains. Regeneration of packets allows for greater distances and more nodes to be usedin the total network design, and dramatically lowers the overall collision rates. In switched networks, eachsegment is an independent collision domain. In shared networks all nodes reside in one, big shared collisiondomain. Easy to install, most switches are self-learning. They determine the Ethernet addresses in use on eachsegment, building a table as packets are passed through the switch. This "plug and play" element makes switchesan attractive alternative to hubs.Switches can connect different networks types (such as Ethernet and Fast Ethernet) or networks of the sametype. Many switches today offer high-speed links, like Fast Ethernet or FDDI that can be used to link the switchestogether or to give added bandwidth to important servers that get a lot of traffic. A network composed of anumber of switches linked together via these fast uplinks is called a "collapsed backbone" network.Dedicating ports on switches to individual nodes is another way to speed access for critical computers. Serversand power users can take advantage of a full segment for one node, so some networks connect high traffic nodesto a dedicated switch port.HubsIn data communications, a hub is the pivot of convergence where data arrives from one or more directions and isforwarded out in one or more directions. A hub usually includes a switch (in telecommunications, a switch is anetwork device that selects a path or circuit for sending a unit of data to its next destination) of some kind. Thedistinction seems to be that the hub is the point where data comes together and the switch is what determineshow and where data is forwarded from the place where data comes together. A hub is a hardware device that actsas a central connecting point and joins lines in a star network configuration.RoutersA router is a device that interconnects two or more computer networks, and selectively interchangespackets of data between them. Each data packet contains address information that a router can use todetermine if the source and destination are on the same network, or if the data packet must betransferred from one network to another. A router is a device whose software and hardware arecustomized to the tasks of routing and forwarding information. A router has two or more networkinterfaces, which may be to different types of network or different network standards.Types of routersBasically these are of two types– 1) Modular: - these routers do not have fixed interfaces. These can be added and removed according to need. 2) Non-modular routers:- These routers have fixed interfaces and these cannot be removed.PortsWe can connect to a Cisco router to configure it, verify its configuration and check the statistics byusing various ports. There are many ports but the most important is the console port.Console Port: -The Technical Zone Page 34
  35. 35. • The console port is usually an RJ-48 connection located at the back of the router. Console is used to configure router when the router is freshly boot and when any time admin wanted to change the running configuration. • We can also connect to the Cisco router by using an auxiliary port, which is the same as the console port. But the auxiliary port also allows us to configure modem commands.Router ComponentsSome of the parts of a cisco router are: Chassis, motherboard, processor, RAM, NVRAM, flash memory,Power supply, Rom etc.ROM: • The ROM in a router contains the bootstrap program that searches for a suitable system image when the router is switched on. When the router is switched on, the ROM performs a Power-on self-test (POST) to check the hardware. POST checks if everything is working in a proper way or not. The ROM also provides a monitor mode that can be used for recovering from a crisis.The Technical Zone Page 35
  36. 36. The information present in the ROM can be erased. ROM contains the basic information which interprets the information to the device.Flash Memory: • Flash memory is an erasable, reprogrammable ROM that holds the system image and the microcode. Flash memory gets its name from the fact that sections of its memory cells are erased in a single action or flash. Flash memory is commonly called Flash. Flash is a variation of EEPROM (Electrically Erasable Programmable Read-Only Memory). The process of erasing and rewriting in EEPROM is slow, while flash is erased and rewritten faster. Flash memory holds the Operating System of a router. The operating system of a Cisco router is IOS (Internetwork Operating System). When a router is switched on, it checks for the compressed form of IOS in Flash memory. If the IOS is present, then it continues else it checks it in the TFTS (Trivial File Transfer Server).RAM: • This is much faster to read from and write to than other kinds of storage, provides catching, buffers network packets, and stores routing table information. RAM contains the running configuration file, which is the current configuration file. All configuration changes are saved to this file unless we explicitly save the changes to the NVRAM. Information in the RAM requires a constant power source to be sustained. When the router is powered down, or there is a power cycle, data stored in RAM ceases to exist. NVRAM is Nonvolatile Random Access Memory. Information in NVRAM is retained in storage when the router is switched off or rebooted.NVRAM • (NVRAM) is the general name used to describe any type of random access memory which does not lose its information when power is turned off. The Startup-configuration is stored in the NVRAM of Router. If the router get reboot it will search the NVRAM for startup-config. If available then the router will copy that Startup-config and put it in running configuration.Internal part of a routerCPU:- • As the function of the CPU, it executes instructions coded in the operating system and its subsystems to perform the basic operations necessary in order to accomplish the functionality of the router, for example, all of the routing functions, network module high-level control, and system initialization.Motherboard Same function as of Computer or Laptop.Router Interface TypesNetwork Module It is type of circuit board on which WIC cards are installed and have permanent FastEthernet or Ethernet slots.WIC Cards Are used to connect the router to other routers in the network or with the Wide areaNetwork like lease lines or frame-relay switch. • Smart serial • SerialFast Ethernet Cards with max-speed of 100Mbps per second. And follow the Ethernet standardsEthernet Cards with max-speed of 10Mbps per second. And follow the Ethernet StandardsBoot SequenceComplete these steps: 1. After you power on the router, the ROM monitor starts first. ROMMON/BOOTSTRAP functions are important at router boot, and complete these operations at boot up:The Technical Zone Page 36