Computer Networks Introduction

Ministry Of higher Education & Scientific Research
Presidency of Slemani Polytechnic University
Kalar Technical Institute
Information Technology Department - Khanaqin
Computer Networks
Assistant Lecture
Mohammad Hassan Husain
2015 - 2016

I
Syllabus
Chapter One: Overview
1. Introduction to Computer Networks ……..…………………….…. …… 1
1.1 Network Applications …..…………………………………….………. 1
1.2 The advantages of computer networks ….…………………….………. 3
1.3 Disadvantages of Computer Networks ………………………... ……… 4
2. Network Components ………………………………………………….. 5
3. Data Communication ………………………………………..………….. 6
3.1 Data Communication Components ……………………………….…… 7
3.2 Data Flow ………………………………………………………….…... 8
4. Network Criteria ……………………………………………………….. 10
5. Network Criteria ……………………………………………………….. 12
Chapter Two: Network Categories
2.1 Network ….…………………………………………….………..….. 13
2.2 Network Categories ……………………………………………….… 13
2.2. A: [Depending on Architecture of the network operating system Software] …....13
1- Peer-to-Peer Networks ………………………………………….. 13
2- Client-Server Networks ………………….………………..…….. 13

II
2.2. B: [Depending on the Size (Area)] ………………………………...... 15
1- Local Area Network (LAN) ………………..…………….……….. 16
2- Metropolitan Area Networks (MAN) ……………..…….………… 17
3- Wide Area Network (WAN) ………………………..…….………. 18
2.3 Network Topologies ……………………………………….….………. 20
2.3.1 Type of Connection ……………………………………..……….. 20
2.3.2 Basic Network Topology Types …………………….….….. .….. 21
2.3.2 .A Physical Topology ………………………………………….. 21
2.3.2. A.1 Mesh Topology ……………..………………………. 21
2.3.2. A.2 Star Topology ……………….………………….…… 23
2.3.2. A.3 Bus Topology …………………………………..…… 24
2.3.2. A.4 Ring Topology ………………………………….….. 24
2.3.2. A.5 Hybrid Topology ……………………………………. 25
2.3.2. B. Logical topology …………………………………………… 26
2.3.2. B.1- Shared Media ……………………………………….. 26
2.3.2. B.2- Token Based ………………………………………… 27
Chapter Three: Transmission Media
3.1 Transmission Media …………………………………………..……….. 29

III
3.1.1 Unguided Media ……………………………………………..….. 29
3.1.2 Guided Media …………………………….………………………. 30
3.1.2.1. Twisted-Pair Cable ...……………………………………… 30
3.1.2.2. Coaxial Cable ……………………………………………...32
3.1.2.3. Fiber-Optic Cable ………………..……………………..… 33
Chapter Four: The OSI and TCP/IP Models
4.1. The OSI Reference Model ………..…………………………………36
4.1.1 Data encapsulation ………………….……………………..….. 37
4.1.2 Layers in the OSI Model ………………………..……………. 39
4.1.2.1The Application Layer (layer 7) ……..…………….….... 39
4.1.2.2 The Presentation Layer (layer 6) …………..……….…. 39
4.1.2.3 The Session Layer (layer 5) ………………………..…. 40
4.1.2.4 The Transport Layer (layer 4) ……………….…….….. 40
4.1.2.5 The Network Layer (layer 3) …………………..….….. 40
4.1.2.6 The Data-Link Layer (layer 2) …………….…………. 41
4.1.2.7 The Physical Layer (layer 1) ………………….………42
4.2 The TCP/IP Reference Model …………………………………….. 44
4.2.1 Layers in the TCP/IP Model ………………………..……….. 46

IV
4.2.1.1 The Application Layer (layer 4) ……………..……….. 46
4.2.1.2 The Transport Layer (layer 3) ………………..………. 47
4.2.1.3 The Internet Layer (layer 2) ……………………..…… 48
4.2.1.4 The Network Access Layer (layer 1) …………………. 50
4.2.2 Header Formats of the Protocols …………………………….. 51
4.2.2.1 Ethernet Frame Format ………………………………. 51
4.2.2.2 IP Header Format …………………………………….. 52
A. TCP Header Format ………………………………….. 53
B. UDP Header Format ……………………………..…… 55
Chapter Five: The Addressing of TCP/IP Protocols
5.1. IPv4 Addresses ……………………………………………………57
5.1.1 IPv4 addresses Classes …………………………………..…. 58
Class A ……………………………………….…..………… 60
Class B ………………………………………..….………… 61
Class C ……………………………………………………... 61
Class D …………………………………………………….. 62
Class E ……………………………………….…………… .. 63
5.1.2 Classless Addressing ……………………………..………. . 63

V
5.2 IPv6 Addresses ……………………………………..………..… 64
5.3. Hardware Address ………………………….…..……………… 65
5.4. Subnet Masks ………………………….…………..…………... 66
References
1- Behrouz A. Forouzan, “ TCP/IP Protocol Suite “, Fourth Edition
2- Sharam Hekmat, “Communication Networks”
3- Andrew S. Tanenbaum, “Computer Networks”, Fourth Edition
4- Behrouz A. Forouzan, “Data Communications and Networking”,
Fourth Edition

Chapter One: Network Introduction Computer Networks
Assistant Lecture Mohammad Hassan Husain Page 1
Chapter One
Network Introduction
1. Introduction to Computer Networks
Computer networks are defined as: “Interconnected collection of
autonomous computers. Two computers are said to be interconnected if they are
able to exchange information”. Or: a network is simply a collection of
intercommunicating computers and peripherals possibly having access to remote
hosts and other computer networks. A network consists of a set of computers:
hosts, connected via a communication subnet, the word “host” refers to an
individual computer connected to the computer, which can communicate with
other hosts via the network.
A network is a set of devices (often referred to as nodes) connected by
communication links. A node can be a computer, printer, or any other device
capable of sending and/or receiving data generated by other nodes on the network.
A network is a combination of hardware and software that sends data from
one location to another. The hardware consists of the physical equipment that
carries signals from one point of the network to another. The software consists of
instruction sets that make possible the services that we expect from a network.
When we communicate, we are sharing information. This sharing can be
local or remote. Between individuals, local communication usually occurs face to
face, while remote communication takes place over distance.

1.1 Network Applications
 Marketing and sales (booking hotel, buying airplane ticket)
 Financial services (Auto Teller Machine (ATM), Exchanging money)
 Manufacturing (multi user work on project simultaneously)
 Electronic messaging (email)
 Directory services (list of files stored in central location to speed up www
search)
 Information services (a www site offering technical specifications for a
product).
 Electronic Data Interchange (EDI) like (purchase order without using paper)
 Teleconferencing (text, voice, video conferencing)
 Cellular telephone (wireless phone connection)
 Displaying weather to decide what to wear using online current weather
conditions.
 Find the least congested route to your destination, displaying traffic video from
webcams.
 Check your bank balance and pay bills electronically.
 Receive and send e-mail, or make an Internet phone call, at an Internet cafe
over lunch.
 Obtain health information and advice from experts all over the world, and post
to a forum to share related health or treatment information.
 Download and upload files.
 Post and share your photographs, home videos, and experiences with friends or
with the world.

1.2 The advantages of computer networks
 File Sharing: The major advantage of a computer network is that is allows
file sharing and remote file access. A person sitting at one workstation of a
network can easily see the files present on the other workstation, provided he
is authorized to do so. It saves the time which is wasted in copying a file from
one system to another, by using a storage device.
 Resource Sharing: Resource sharing is also an important benefit of a
computer network. For example, if there are four people in a family, each
having their own computer, they will require four modems (for the Internet
connection) and four printers, if they want to use the resources at the same
time. A computer network, on the other hand, provides a cheaper alternative
by the provision of resource sharing. In this way, all the four computers can be
interconnected, using a network, and just one modem and printer can
efficiently provide the services to all four members. The facility of shared
folders can also be availed by family members.
 Increased Storage Capacity: As there is more than one computer on a
network which can easily share files, the issue of storage capacity gets
resolved to a great extent. A standalone computer might fall short of storage
memory, but when many computers are on a network, memory of different
computers can be used in such case. One can also design a storage server on
the network in order to have a huge storage capacity.
 Increased Cost Efficiency: There are many software available in the
market which are costly and take time for installation. Computer networks
resolve this issue as the software can be stored or installed on a system or a
server and can be used by the different workstations.

Figure 1: Modern networks can contain several components for allowing data
and resource sharing.
1.3 Disadvantages of Computer Networks
Following are some of the major disadvantages of computer networks.
 Security Issues: One of the major drawbacks of computer networks is the
security issues involved. If a computer is a standalone, physical access
becomes necessary for any kind of data theft. However, if a computer is on a
network, a computer hacker can get unauthorized access by using different
tools. In case of big organizations, various network security software are used
to prevent the theft of any confidential and classified data.
 Rapid Spread of Computer Viruses: If any computer system in a
network gets affected by computer virus, there is a possible threat of other
systems getting affected too. Viruses get spread on a network easily because

of the interconnectivity of workstations. Such spread can be dangerous if the
computers have important database which can get corrupted by the virus.
 Expensive Set Up: The initial set up cost of a computer network can be
high depending on the number of computers to be connected. Costly devices
like routers, switches, hubs, etc., can add up to the bills of a person trying to
install a computer network. He will also have to buy NICs (Network Interface
Cards) for each of the workstations, in case they are not inbuilt.
 Dependency on the Main File Server: In case the main File Server of
a computer network breaks down, the system becomes useless. In case of big
networks, the File Server should be a powerful computer, which often makes
it expensive.
2- Network Components:
Network components are used to connect devices on different networks,
to create and connect multiple networks or subnets. The components include:
 NIC: (Network Interface Card) is used to enable a network device, such as
a computer or other network equipment, to connect to a network.
 Repeater: A repeater is an inexpensive solution that is at the OSI physical
layer and enables a network to reach users in distant portions of a building.A
repeater connects two or more cable segments and retransmits any incoming
signal to all other segments.
 Hubs or Switches: A hub is a central network device that connects
network nodes such as workstation and servers in a star topology. A hub
may also be referred to as a concentrator, which is a device that can have
multiple inputs and outputs all active at one time.

 Bridge: A bridge is a network device that sends information between two
LANs.
 Router: Routers are devices that direct traffic between hosts.
 Servers: A computer or device on a network that manages network
resources. There are many different types of servers such as File server,
Print server, Database server.
3- Data Communication
Data communication is the exchange of data (in the form of 0s and 1s)
between two devices via some form of transmission medium (wire or wireless).
 The effectiveness of a data communication system depends on three fundamental
characteristics, as illustrated in below:
1- Delivery: the system must deliver data to the correct destination.
2- Accuracy: the system must deliver data accurately.
3- Timeliness: the system must deliver data in a timely manner. Data delivered
late are useless. In case of video, audio and voice data, timely delivery mean
delivering data as they are produced.

3.1 Data Communication Components
 A data communication system is made up of five components, they are:
 Message, sender, receiver, medium, protocols
 Message: the message is the information (data) to be communicated. It can
consist of text, numbers, pictures, sound, or video, etc…
 Sender: the sender is the device that sends the data message. It can be
computer, workstation, telephone handset, video camera, and so on.
 Receiver: the receiver is the device that receives the data message. It can be
computer, workstation, telephone handset, television, and so on.
 Medium: the transmission medium is the physical path by which a message
travels from sender to receiver. It can consist of twisted pair wire, coaxial
cable, fiber optic cable, laser, or radio waves (satellite micro wave).
 Protocol: is a set of rules that govern data communication. It represents an
agreement between the communicating devices. Without a protocol, two
devices may be connected but not communicating.

3.2 Data Flow
Communication between two devices can be simplex, half-duplex, or full-
duplex as shown in Figure
Figure 2: Data flow (A-Simplex, B-Half-Duplex, C-Full-Duplex)
A- Simplex:
In simplex mode, the communication is unidirectional, as on a one-way street.
Only one of the two devices on a link can transmit; the other can only receive.
Keyboards and traditional monitors are examples of simplex devices. The
keyboard can only introduce input; the monitor can only accept output.

B- Half-Duplex
In half-duplex mode, each station can both transmit and receive, but not at the
same time. When one device is sending, the other can only receive, and vice versa.
C- Full-Duplex
In full-duplex (called duplex), both stations can transmit and receive
simultaneously. The full-duplex mode is like a two-way street with traffic flowing
in both directions at the same time. One common example of full-duplex
communication is the telephone network. When two people are communicating by
a telephone line, both can talk and listen at the same time. The capacity of the
channel, however, must be divided between the two directions.

Network Bandwidth and Throughput
Bandwidth: Number of bits per second that can be sent by a device across a
particular transmission medium.
Throughput is how many bits are actually transferred between two computers in a
given time.
Two points to consider when comparing throughput to bandwidth:
- Throughput rate may vary over time due to network conditions; bandwidth
does not vary over time.
- Bandwidth defines the speed of a single link; throughput measures the speed
of the end-to-end connection.
- Examples of Throughput

Factors that affect Throughput
1- Speed and current workload of the computers.
2- Analog limitation.
2- Multi-User considerations.
3- Congestion level in the network.
Calculating Data Transfer Time: Two Methods
Calculating Data Transfer Time: Four Examples from the “Examples of
Throughput” figure

4- Network Criteria
A network must be able to meet a certain number of criteria. The most
important of these are performance, reliability, and security.
1- Performance
Can be measured in many ways, including transit time and response time.
Transit time is the amount of time required for a message to travel from one
device to another. Response time is the time between inquiry and a response. The
performance of a network depends on a number of factors, including:
* Number of users: having a large number of concurrent users can slow
response time in a network not designed to coordinate heavy traffic loads.
* Type of transmission medium: the medium defines the speed at which data
can travel through a connection (the data rate). (i.e. 10 mbps, 100 mbps, 1000
mbps, 10000 mbps).
* Hardware: the types of hardware included in a network affect both the speed
and capacity of transmission.
* Software: the software used to process data at the sender, receiver, and
intermediate nodes also affects network performance.
2- Reliability
Measured by frequency of failure, the time it takes a link to recover from a
failure, and the network’s robustness in catastrophe.
3- Security
Network security issues include protecting data from unauthorized access,
protecting data from damage and development, and implementing policies and
procedures for recovery from breaches and data losses.

Chapter Two: Networking Basics Computer Networks
Assistant lecturer Mohammad Hassan Husain Page 13
Chapter Two
Networking Basics
2.1 Network
A network is a set of devices (often referred to as nodes) connected by
communication links. A node can be a computer, printer, or any other devices
capable of sending and/or receiving data generated by other nodes on the network.
2.2 Network Categories
All networks consist of the same three basic elements, as follows:
• Protocols: A protocol is a set of rules or standards designed to enable
computers to connect with one another and to exchange information with
as little error as possible.
• Transmission media: media that enable all networking elements to
interconnect.
• Network services: resources that are shared with all network users.
2.2. A: [Depending on Architecture of the network operating system software]
There are two main types of network are:
• Peer-to-Peer Networks
• Client-Server Networks
In a peer-to-peer network, the connected computers have no centralized
authority. From an authority viewpoint, all of these computers are equal. In other
words, they are peers. If a user of one computer wants access to a resource on
another computer, the security check for access rights is the responsibility of the
computer holding the resource.

Each computer in a peer-to-peer network can be both a client that requests
resources and a server that provides resources. This is a great arrangement,
provided the following conditions are met:
 Each user is responsible for local backup.
 Security considerations are minimal.
 A limited number of computers are involved.
Peer-to-peer networks present some challenges. For example, if you have a
large business with hundreds of computers, it could take a long time to locate the
file you need. Also, it can be difficult to remember where you stored a file. Finally,
because security is not centralized, users and passwords must be maintained
separately on each machine. Passwords may be different for the same users on
different machines.
This type of network is usually associated with smaller businesses where
security is not an issue.
Figure 1: A peer-to-peer network
In a Client-Server Networks, uses a network operating system designed to
manage the entire network from a centralized point, which is the server. Clients
make requests of the server, and the server responds with the information or access
to a resource.

Client/server networks have some definite advantages over peer-to-peer
networks. For one thing, the network is much more organized. It is easier to find
files and resources because they are stored on the server. Also, client/server
networks generally have much tighter security. All usernames and passwords are
stored in the same database (on the server). You would not have to enter a separate
password for each document that you want to access – making much more efficient
use of your time. Finally, client/server networks have better performance than a
peer to peer network.
Client-server networks are usually found in larger businesses where security is
an issue. However, a client-server network can also work for your small business.
Figure 2: A Client-Server Network
2.2. B: [Depending on the Size (Area)]
One way to characterize networks is according to their size (Area). Two well
Known examples are LANs (Local Area Networks) and WANs (Wide Area
Networks). Other networks are classified as MANs (Metropolitan Area Networks).

Figure 3: Network classification according to type of size or area
1-Local Area Network (LAN)
A local area network (LAN) is usually privately owned and links the devices
in a single office, building, or campus (see Figure 4). Depending on the needs of an
organization and the type of technology used, a LAN can be as simple as two PCs
and a printer in someone's home office; or it can extend throughout a company and
include audio and video peripherals. Currently, LAN size is limited to a few
kilometers.
The first LAN was limited to a range (from a central point to the most distant
computer) of 185 meters (about 600 feet) and no more than 30 computers. Today’s
technology allows a larger LAN, but practical administration limitations require
dividing it into small, logical areas called workgroups. A workgroup is a collection
of individuals (a sales department, for example) who share the same files and
databases over the LAN.

Figure 4: A small LAN network
2-Metropolitan Area Networks (MAN)
A metropolitan area network (MAN) is a network with a size between a LAN
and a WAN. It normally covers the area inside a town or a city.. It may be a single
network such as a cable television network, or it may be a means of connecting a
number of LAN into a larger network so that resources may be shared LAN-to-
LAN as well as device-to-device. For example, a company can use a MAN to
connect the LANs in all of its offices throughout a city. Another example is the
cable TV network that originally was designed for cable TV.

Figure 5: Metropolitan area network (MAN)
3- Wide Area Network (WAN)
A wide area network (WAN) provides long-distance transmission of data,
image, audio, and video information over large geographic areas that may
comprise a country, a continent, or even the whole world.
A WAN is any network that crosses metropolitan, regional, or national
boundaries. Most networking professionals define a WAN as any network that uses
routers and public network links. The Internet is actually a specific type of WAN.
The Internet is a collection of networks that are interconnected and, therefore, is
technically an internetwork (Internet is short for the word 'International network').
A WAN can be centralized or distributed. A centralized WAN consists of a central
computer (at a central site) to which other computers and dumb terminals connect.
The Internet, on the other hand, consists of many interconnected computers in
many locations. Thus, it is a distributed WAN.

Figure 6: Wide Area Network (WAN)
WANs differ from LANs in the following ways:
 WANs cover greater distances.
 WAN speeds are slower.
 WANs can use public or private network transports; LAN primarily use
private network transports.

2.3- Network Topologies
Network topology is the layout pattern of interconnections of the various
elements (links, nodes, etc.) of a computer network. Network topologies may be
physical or logical.
Physical topology means the physical design of a network including the
devices, location and cable installation.
Logical topology is the way that the signals act on the network media, or the
way that the data passes through the network from one device to the next without
regard to the physical interconnection of the devices.
2.3.1 Type of Connection
A network is two or more devices connected through links. A link is a
communications pathway that transfers data from one device to another. For
communication to occur, two devices must be connected in some way to the same
link at the same time. There are two possible types of connections: point-to-point
and multipoint.
1. Point-to-Point
A point-to-point connection provides a dedicated link between two devices.
Point-to-point networks consist of many connections between individual pairs of
machines. To go from the source to the destination, a data on this type of network
may have to first visit one or more intermediate machines.
2. Multipoint (Broadcast)
A multipoint (Broadcast) has a single communication channel that is shared
by all the machines on the network. In Broadcast, the data sent by any machine is
received by all the others.

2.3.2 Basic Network Topology Types
2.3.2. A- Physical Topology
A topology is basically a map of a network. The physical topology of a
network describes the layout of the cables and workstations and the location of all
network components.
There are four basic topologies possible: mesh, star, bus, and ring. Each
topology has its advantages and drawbacks. You should balance the following
considerations when choosing a physical topology for your network:
 Cost.
 Ease of installation.
 Ease of maintenance.
 Cable fault tolerance.
Figure 7: Category of network topology
2.3.2. A.1- Mesh Topology
In a mesh topology, every device has a dedicated point-to-point link to every
other device. The term dedicated means that the link carries traffic only between
the two devices it connects. To find the number of physical links in a fully
connected mesh network with n nodes, we first consider that each node must be

connected to every other node. Node 1 must be connected to n – 1 nodes, node 2
must be connected to n – 1 nodes, and finally node n must be connected to n - 1
nodes. We need n(n - 1) physical links. However, if each physical link allows
communication in both directions (Full Duplex mode), we can divide the number
of links by 2. In other words, we can say that in a mesh topology, we need
n(n -1) /2
duplex-mode links.
To accommodate that many links, every device on the network must have n – 1
input/output (IO) ports (see Figure 8) to be connected to the other n - 1 stations.
Figure 8: A fully connected mesh topology
A mesh offers several advantages over other network topologies:
The use of dedicated links guarantees that each connection can carry its own data
load, robust, privacy or security, fault identification and fault isolation easy.
The main disadvantages of a mesh are related to the amount of cabling and
the number of I/O ports required.

One practical example of a mesh topology is the connection of telephone
regional offices in which each regional office needs to be connected to every other
regional office.
2.3.2. A.2- Star Topology
In a star topology, each device has a dedicated point-to-point link only to a
central controller, usually called a hub. The devices are not directly linked to one
another. Unlike a mesh topology, a star topology does not allow direct traffic
between devices. The controller acts as an exchange: If one device wants to send
data to another, it sends the data to the controller, which then relays the data to the
other connected device (see Figure 9).
Figure 9: A star topology connecting
Advantages of this topology:
1- A star topology is less expensive than a mesh topology.
2- It easy to install and reconfigure.
3- Include robustness. If one link fails, only that link is affected. All other links
remain active.
4- Easy fault identification and fault isolation.
Disadvantages: If the hub (or centralized connection point) malfunctions, the
entire network can fail.

2.3.2. A.3- Bus Topology
The preceding examples all describe point-to-point connections. A bus
topology, on the other hand, is multipoint. Bus topology networks require that all
computers, or nodes, connect to the same cable. When a computer sends data, that
data is broadcast to all nodes on the network. (see Figure 10).
Figure 10: A bus topology connecting
Advantages of a bus topology include ease of installation. It uses less cabling
than mesh or star topologies.
Disadvantages: It can therefore be difficult to add new devices. In addition, a
fault or break in the bus cable stops all transmission.
2.3.2. A.4- Ring Topology
Ring topologies do not have a central connection point. Instead, a cable
connects one node to another. When a node sends a message, the message is
processed by each computer in the ring. If a computer is not the destination node, it
will pass the message to the next node, until the message arrives at its destination.
If the message is not accepted by any node on the network, it will travel around the
entire ring and return to the sender (see Figure 11).

Figure 11: A ring topology connecting
The advantages of this topology:
1. easy to install and reconfigure.
2. To add or delete a device requires changing only two connections.
3. Fault isolation is simplified.
The disadvantages of this topology
Unidirectional traffic can be a disadvantage. In a simple ring, a break in the
ring (such as a disabled station) can disable the entire network. This weakness can
be solved by using a dual ring or a switch capable of closing off the break.
2.3.2. A.5- Hybrid Topology
Larger networks combine the bus, star and ring topologies. This combination
allows expansion even in enterprise networks. Two common examples are star ring
and star bus.

2.3.2. B. Logical topology (also referred to as signal topology)
A logical topology defines the logical layout of a network. This specifies how
the elements in the network communicate with each other and how information is
transmitted. Logical topologies are bound to network protocols and describe how
data is moved across the network.
The two main logical topologies are Shared Media and Token Passing
topology. These are each associated with different types of media-access methods,
which determine how a node gets to transmit information along the network.
2.3.2. B.1- Shared Media
In a shared media topology, all the systems have the ability to access the
physical layout whenever they need it. The main advantage in a shared media
topology is that the systems have unrestricted access to the physical media. But,
the main disadvantage to this topology is collisions. If two systems send
information out on the wire at the same time, the packets collide and kill both
packets. Ethernet is an example of a shared media topology. To help avoid the
collision problem, Ethernet uses a protocol called Carrier Sense Multiple
Access/Collision Detection (CSMA/CD). CSMA/CD is the method used in
Ethernet networks for controlling access to the physical media by network nodes.
CSMA/CD process can be described as follows: Listen to see whether the wire is
being used.
• If the wire is busy, wait.
• If the wire is quiet, send.
• If a collision occurs while sending, stop wait a specific amount of time, and
send again.

Figure 13: CSMA/CD Process
2.3.2. B.2- Token Passing
The Token Passing topology works by using a token to provide access to the
physical media. In a Token Passing network, there is a token that travels around
the network. When a system needs to send out packets, it grabs ‫(يستولي‬)‫يمسك‬ the
token off of the wire, attaches it to the packets that are sent, and sends it back out
on the wire. As the token travels around the network, each system examines the
token. When the packets arrive at the destination systems, those systems copy the
information off of the wire and the token continues its journey until it gets back to
the sender. When the sender receives the token back, it pulls )‫(يسكب‬ the token off
of the wire and sends out a new empty token to be used by the next machine.
Token Passing networks do not have the same collision problems that Ethernet-
based networks do because of the need to have possession )‫(تسكتبو‬ of the token to
communicate. However, one problem that does occur with Token Passing
networks is latency ‫(التأخير‬)‫االنتظكر‬ . Because each machine has to wait until it can
use the token, there is often a delay in when communications actually occur. Token

Passing network are typically configured in physical ring topology because the
token needs to be delivered back to the originating machine for it to release. The
ring topology best facilitates this requirement.
Figure 14: Token Ring Network

Chapter Three: Transmission Media Computer Networks
Chapter Three
Transmission Media
3.1 Transmission Media
To transmit data, a medium must exist, usually in the form of cables or
wireless methods. A transmission medium can be broadly defined as anything
that can carry information from a source to a destination. The transmission medium
is usually free space, metallic cable, or fiber-optic cable. The information is usually
a signal that is the result of a conversion of data from another form. In
telecommunications, transmission media can be divided into two broad categories:
guided and unguided. Guided media include twisted-pair cable, coaxial cable, and
fiber-optic cable. Unguided medium is free space.
3.1.1 Unguided Media
Unguided media (free space) transport electromagnetic waves without the use
of a physical conductor. Wireless waves can be classified as radio waves,
microwaves, or infrared waves. Radio waves are omnidirectional; microwaves
are unidirectional. Microwaves are used for cellular phone, satellite, and wireless
LAN communications. Infrared waves are used for short-range communications
such as those between a PC and a peripheral device.

3.1.2 Guided Media
Guided media, which are those that provide a conduit from one device to
another, include twisted-pair cable, coaxial cable, and fiber-optic cable. The type
of cable chosen for a network is related to the network's location, data rate, cost
and distance. A signal traveling along any of these media is directed and contained
by the physical limits of the medium. Twisted-pair and coaxial cable use metallic
(copper) conductors that accept and transport signals in the form of electric current.
Optical fiber is a cable that accepts and transports signals in the form of light.
3.1.2.1. Twisted-Pair Cable
This cable type is the most common today. It is popular for several reasons:
 It’s cheaper than other types of cabling.
 It’s easy to work with.
Twisted-pair cable is available in two basic types:
1. Shielded twisted-pair (STP): Shielded twisted-pair copper wire is
protected from external electromagnetic interference by a metal sheath
wrapped around the wires; STP is harder to install and maintain than UTP.

2. Unshielded twisted-pair (UTP): Unshielded twisted-pair cable is the most
common type of twisted-pair wiring; it is less expensive than STP, but it is
less secure and is prone to electromagnetic interference.
The most common UTP connector is Registered Jack-45. RJ-45 connectors
are commonly used on certain types of Ethernet and token-ring networks. The
connector holds up to eight wires, and is used with twisted-pair wire. To attach an
RJ-45 connector to a cable, the connector must be crimped using a tool called a
crimper.
Most telephones connect with an RJ-11 connector. The RJ-11 has four wires
or two pairs).

The EIA/TIA (Electronic Industry Association/Telecommunication Industry
Association) has established standards of UTP and rated six categories of wire
3.1.2.2. Coaxial Cable
Coaxial cable, known as coax (pronounced "co-axe"), is a high-capacity cable
used for video and communication networks. Coaxial cable has remained in
common networking use because cable companies are often a preferred choice for
high-speed Internet access. Coaxial cable contains a signal wire at the center,
surrounded by a metallic shield that serves as a ground. The shield is either braided
or solid, and is wrapped in plastic.
Coaxial cables are categorized by their Radio Guide (RG) ratings. Each RG
number denotes a unique set of physical specifications.

To connect coaxial cable to devices, we need coaxial connectors. The most
common type of connector used today is the Bayonet Neil-Concelman (BNC).
3.1.2.3. Fiber-Optic Cable
Fiber optic cables consist of two small glass strands: One strand sends and one
receives. These strands are called the core. Each core is surrounded by glass
cladding. Each core and cladding element is wrapped with a plastic reinforced with
Kevlar fibers. Laser transmitters send the modulated light pulses and optical
receivers receive them.
Fiber optic cable can accommodate data transmissions much faster than
coaxial or twisted-pair cable. Fiber optic lines can transmit data in the gigabits per
second range. Because they send data as pulses of light over threads of glass, the
transmissions can travel for miles without a signal degradation. No electrical
signals are carried over the fiber optic line, so the lines are free of electromagnetic
interference and are extremely difficult to tap.

Following are the two major types of fiber optic cable:
 Single-mode: uses a specific light wavelength. The cable's core diameter
is 8 to 10 microns. It permits signal transmission at extremely high bandwidth and
allows very long transmission distances (up to 70 km, or 43 miles). Single mode
fiber is often used for intercity telephone trunks and video applications.
 Multi-mode: uses a large number of frequencies (or modes). The cable's
core is larger than that of single-mode fiber, usually 50 microns to 100 microns,
and it allows for the use of inexpensive light sources. It is used for short to medium
distances (less than 200 m, or 656 feet). Multi-mode fiber is the type usually
specified for LANs and WANs.

Fiber optic cable is expensive, and installation can be tedious and costly.
Attaching connectors to fibers used to involve a tedious process of cutting and
polishing the ends of the glass strands, and then mounting them into the
connectors. Modern tools and newer connectors cut and polish in one step.

Chapter Four: The OSI and TCP/IP Models Computer Networks
Chapter Four
The OSI and TCP/IP Models
Reference Model
We have discussed layered networks; it is time to look at some examples. In
the next two sections we will discuss two important network architectures, the OSI
reference model and the TCP/IP reference model.
4.1. The OSI Reference Model
In the last 1970s the International Standards Organization (ISO) adopted
the Open System Interconnection (OSI) model. The OSI model breaks down the
many tasks involved in moving data from one host to another. The OSI are divided
into seven smaller group, the seven groups are called layers.
Figure 4.1: The OSI model
An open system is a set of protocols that allows any two different systems to
communicate regardless of their underlying architecture. The purpose of the OSI
model is to show how to facilitate communication between different systems
without requiring changes to the logic of the underlying hardware and software.
The OSI model is not a protocol; it is a model for understanding and designing a
network architecture that is flexible, robust, and interoperable.

4.1.1 Data encapsulation
When the source host sends data to destination host, the application data is
sent down by source host through the layers in protocol stack. Each layer adds a
control information as a header to the data and may be add a trailer to the data.
This control information added to the data called Protocol Data Unit (PDU), and
the process of adding the PDU to the data (encoding data with PDU) called
encapsulation. On other hand, the process of extracting the data from PDU by the
destination host (decoding data from PDU) in the specific layer that corresponds to
the same layer in source host is called de-encapsulation. Figure 4.2 shows us the
data encapsulation for OSI layers.

Figure 4.2: Data Encapsulation
The above figure shows the data encapsulation for physical layer is called
Bits, the data encapsulation in data-link layer is called Frame, whereas the data
encapsulation in network layer is called Packet or Datagram, and the data
encapsulation for Transport layer is called Segment.

4.1.2 Layers in the OSI Model
In this section we briefly describe the functions of each layer in the OSI model.
4.1.2.1The Application Layer (layer 7)
The application layer enables the user, whether human or software, to access
the network. It provides user interfaces and support for services such as electronic
mail, remote file access and transfer, shared database management, and other types
of distributed information services. Specific services provided by the application
layer include the following:
 File transfer, access, and management (FTAM): This application allows a
user to access files in a remote host (to make changes or read data), to retrieve
files from a remote computer for use in the local computer, and to manage or
control files in a remote computer locally.
 E-mail services: This application provides the basis for e-mail forwarding
and storage.
4.1.2.2 The Presentation Layer (layer 6)
The presentation layer is concerned with the syntax and semantics of the
information exchanged between two systems. The primary job of the Presentation
layer is to ensure that the message gets transmitted in a language or syntax that the
receiving computer can understand. Specific responsibilities of the presentation
layer include the following:
 Encryption: To carry sensitive information a system must be able to assure
privacy. Encryption means that the sender transforms the original
information to another form and sends the resulting message out over the

network. Decryption reverses the original process to transform the message
back to its original form.
 Compression: Data compression reduces the number of bits contained in the
information. Data compression becomes particularly important in the
transmission of multimedia such as text, audio, and video.
4.1.2.3 The Session Layer (layer 5)
The session layer allows users on different machines to establish sessions
between them, that’s mean the session layer allows two systems to enter into a
dialog. The session layer can allow traffic to go in both directions at the same time
(Full-Duplex), or in only one direction at a time (Half-Duplex).
4.1.2.4 The Transport Layer (layer 4)
The basic function of the transport layer is to accept data from the session
layer, split it up into smaller units if need be, pass these to the network layer, and
ensure that the pieces all arrive correctly at the other end. The transport layer
ensures that the whole message arrives intact and in order, overseeing both error
control and flow control at the source-to-destination level.
4.1.2.5 The Network Layer (layer 3)
The Network layer is responsible for the source-to-destination delivery of a
packet, possibly across multiple networks (links). It ensures that each packet gets
from its point of origin to its final destination. In other word, the Network layer is
responsible for routing the packet based on its logical address.

4.1.2.6 The Data-Link Layer (layer 2)
The main task of the data link layer is to take a raw transmission facility and
transform it into a line that appears free of undetected transmission errors to the
network layer. The packet is encapsulated into a frame. The data link layer is
responsible for moving frames from one node to the next. And other
responsibilities of the data link layer include the following:
 Framing: The data link layer divides the stream of bits received from the
network layer into manageable data units called frames.
 Physical addressing: If frames are to be distributed to different systems on
the network, the data link layer adds a header to the frame to define the
sender and/or receiver of the frame. If the frame is intended for a system
outside the sender's network, the receiver address is the address of the device
that connects the network to the next one.
 Flow control: If the rate at which the data are absorbed by the receiver is less
than the rate at which data are produced in the sender, the data link layer
imposes a flow control mechanism to avoid overwhelming the receiver.
 Error control: The data link layer adds reliability to the physical layer by
adding mechanisms to detect and retransmit damaged or lost frames. It also
uses a mechanism to recognize duplicate frames. Error control is normally
achieved through a trailer added to the end of the frame.
 Access control: When two or more devices are connected to the same link,
data link layer protocols are necessary to determine which device has control
over the link at any given time.
Protocols at this layer aid in the addressing and error detection of data being
transferred. The Data-Link layer is made up of two sublayers: the Logical Link
Control (LLC) sublayer and the Media Access Control (MAC) sublayer. Each

sublayer provides its own services. The LLC sublayer is the interface between
Network layer protocols and the media access method, for example, Ethernet or
Token Ring. The MAC sublayer handles the connection to the physical media,
such as twisted-pair or coaxial cabling.
4.1.2.7 The Physical Layer (layer 1)
The physical layer coordinates the functions required to carry a bit stream
over a physical medium. It deals with the mechanical and electrical specifications
of the interface and transmission media. It also defines the procedures and
functions that physical devices and interfaces have to perform for transmission to
occur. The physical layer is also concerned with the following:
 Physical characteristics of interfaces and media: The physical layer
defines the characteristics of the interface between the devices and the
transmission media. It also defines the type of transmission media (see
Chapter 3).
 Representation of bits: The physical layer data consists of a stream of bits
(sequence of 0s or 1s) with no interpretation. To be transmitted, bits must be
encoded into signals (electrical or optical). The physical layer defines the
type of encoding (how 0s and 1s are changed to signals).
 Data Rate: The transmission rate (the number of bits sent each second) is
also defined by the physical layer. In other words, the physical layer defines
the duration of a bit, which is how long it lasts.
 Line Configuration: The physical layer is concerned with the connection of
devices to the media. In a point-to-point configuration, two devices are
connected together through a dedicated link. In a multipoint configuration, a
link is shared between several devices.

 Physical Topology: The physical topology defines how devices are
connected to make a network. Devices can be connected using a mesh
topology (every device connected to every other device), a star topology
(devices are connected through a central device), a ring topology (each
device is connected to the next, forming a ring), or a bus topology (every
device on a common link).
 Transmission Mode: The physical layer also defines the direction of
transmission between two devices: simplex, half-duplex, or full-duplex.

4.2. The TCP/IP Reference Model
TCP/IP is a set of protocols that enable communication between computers.
The TCP/IP protocol is the most widely used. Part of the reason is that TCP/IP is
the protocol of choice on the Internet. Another reason for TCP/IP’s popularity is
that it is compatible with almost every computer in the world. The TCP/IP stack is
supported by current versions of all the major operating systems and network
operating systems.
TCP/IP was developed using the Department of Defense (DoD) reference
model.
Figure (4.3) TCP/IP and OSI model

Figure (4.4): The TCP/IP Protocols
Figure (4.5): The TCP/IP Addressing

4.2.1 Layers in the TCP/IP Model
In this section we briefly describe the protocols of each layer in the TCP/IP model.
4.2.1.1 The Application Layer (layer 4)
The application layer generates the data to be sent over the network and
processes the corresponding data received over the network. It contains all the
higher-level protocols such as virtual terminal (TELNET), File Transfer Protocol
(FTP), Simple Mail Transfer Protocol (SMTP), Dynamic Host Configuration
Protocol (DHCP), Domain Name System (DNS) and Hypertext Transfer Protocol
(HTTP).
 The TELNET Protocol allows a user on one machine to log into a distant
machine and work there.
 The FTP is the protocol that defines how a file can be transferred from one
host to another.
 The SMTP protocol is used to send mail across the internet.
 The DHCP protocol enables host systems in a TCP/IP network to be
configured automatically for the network as they boot.
 The DNS protocol is the Internet’s mechanism for linking all the host names
and IP addresses on the Internet. All the URLs (Uniform Resource Locator)
that you need to get resolution for on the Internet are in a DNS database
somewhere.

 The HTTP protocol is a set of rules for exchanging files on the Internet. This
is the protocol that your Web browser uses when surfing (‫)تتصفح‬ the Internet.
4.2.1.2 The Transport Layer (layer 3)
The protocols at the Transport layer deliver data to and receive data from the
Transport layer protocols of other hosts. The Transport layer of the TCP/IP
protocol suite consists of only two protocols, Transmission Control Protocol (TCP)
and User Datagram Protocol (UDP).
Transmission Control Protocol (TCP)
TCP protocol provides connection-oriented, reliable communication.
Connection-oriented means that allow a data stream originating on one machine
to be delivered without error on any other machine in the internet. Reliable means
that an acknowledgment will be sent back to the sending host throughout the
communication to verify receipt of the packets.
TCP is slower and typically used for transferring large amounts of data to
ensure that the data won’t have to be sent again.
TCP is used for many applications such as FTP, HTTP, and SMTP etc.

User Datagram Protocol (UDP)
User Datagram Protocol (UDP) is the protocol used at the Transport layer for
connectionless, non-guaranteed communication. Connectionless means that the
communication that occurs without a connection first being set up. Unlike TCP,
UDP do not set up a connection and do not use acknowledgments.
UDP is an unreliable, "unreliable" merely means that there are no techniques
in the protocol for verifying that the data reached the other end of the network
correctly.
UDP is faster and typically used for transferring small amounts of data.
4.2.1.3 The Internet Layer (layer 2)
The Internet layer contains the protocols that are responsible for addressing
and routing of packets (The process of determining which is the next path to send a
packet so that it gets to its destination is called routing). The Internet layer contains
several protocols, including:
 Internet Protocol (IP)
The Internet Protocol (IP) is the primary protocol at the Internet layer of the
TCP/IP stack. IP is responsible for:
 IP addressing: The IP addressing conventions are part of the IP protocol.
 Host-to-Host communications: IP determines the path a packet must take,
based on the receiving host’s IP address.
 Packet formatting: IP assembles data into units known as IP datagrams or
packets.
 Fragmentation: If a datagram is too large for transmission over the network
media, IP on the sending host breaks the datagram into smaller fragments

(packets). IP on the receiving host then reconstructs the fragments (packets)
into the original datagram.
 Address Resolution Protocol (ARP).
Address Resolution Protocol (ARP) is a protocol used for resolution of
network layer addresses into data-link layer address, that’s mean it’s used to
convert an IP address to a physical address (mac address). ARP protocol
assists IP in directing packet to the appropriate receiving host by mapping
MAC addresses (48 bits long) to known IP addresses (32 bits long).
 Reverse Address Resolution Protocol (RARP).
The protocol which asks for translation from an IP address to a hardware
address is called an ARP, while the reversal protocol for translating
hardware addresses to IP addresses is called Reverse Address Resolution
Protocol (RARP).
 Internet Control Message Protocol (ICMP)
This protocol is part of the Internet Layer and uses the IP datagram
delivery facility to send its messages. It is used for checking remote hosts.
The ping and traceroute commands use this message.
 Internet Group Message Protocol (IGMP)
IGMP is a protocol that enables one host to send one stream of data to
many hosts at the same time.

4.2.1.4 The Network Access Layer (layer 1)
The Network Access layer, also called the network interface layer, is the
lowest layer of the TCP/IP protocol hierarchy. The protocols in this layer provide
the means for the system to deliver data to the other devices on a directly attached
network. The access layer is also responsible for retransmissions of packets
received in error over the link.
Figure (4.6): Communication at the networks

4.2.2 Header Formats of the Protocols
The PDU for each layer in TCP/IP is explained in the data encapsulation. This
section explains the related headers for each layer used in the proposed project.
4.2.2.1 Ethernet Frame Format
The Ethernet frame size is 1518 bytes. The Ethernet frame consist of three
parts as shown in Figure (4.7), which are the header, payload, and trailer
Figure (4.7): Ethernet frame format
The first six bytes are a destination MAC address, while the next six bytes
represent the source MAC address, and the frame type that determines the layer 3
protocol, it is represented by the next 2 bytes. After these frame header fields, the
frame payload has a size between 46 bytes to a maximum size for the frame
header, which is 1500 bytes. Finally the frame trailer that contains the cyclic
redundancy check (CRC) field is used for error detection.

4.2.2.2 IP Header Format
The IP header is the first part from the frame payload, if the frame type has
value 0x0800 which represents the IP protocol. The IP header has size 20 bytes if
no option is present. Figure (4.8) shows the IP header format.
Figure (4.8): IP header
The IP header fields are explained below:
 VER: The version of IP used, for example 4 for IPV4, 6 for IPV6.
 HLEN: Represent the header length in 32-bit words.
 Service type: contains an 8-bit binary value that is used to determine the
priority of each packet.
 Total Length: contains the total length of the IP datagram in bytes. Because
this entry only has two bytes, the maximum IP datagram length is 65,535 bytes.
 Identification: Contains an integer that identifies the current packet.

 Flags: Specifies whether fragmentation should occur.
 Fragmentation Offset: Indicates the position of the fragment’s data relative to
the beginning of the data in the original datagram. It allows the destination IP
process to properly reconstruct the original packet.
 Time to Live: A counter that is decremented by one each time the packet is
forwarded. A packet with 0 in this field is discarded.
 Protocol: The upper layer protocol that is the source or destination of the data.
For example, value 1 represents ICMP, 2 for IGMP, 6 for TCP, and 17 for
UDP.
 Header Checksum: This field is used to verify the IP header correctness.
 Source IP Address: The IP address of the sending host.
 Destination IP Address: The IP address of the receiving host.
 Options: Used for network testing, debugging, security, and more.
A. TCP Header Format
If the protocol type in IP header has value 6, then the packet is a TCP packet.
TCP header has size 20 bytes or 24 with options field. Figure (4.9) shows the TCP
header that comes after IP header

Figure (4.9): TCP header
The TCP header fields are explained below:
 Source Port Number: The port number of the source process.
 Destination Port Number: The port number of the process running in the
destination host.
 Sequence Number: Identifies the byte in the stream of data from the sending
TCP to the receiving TCP. It is the sequence number of the first byte of data in
this segment represents.
 Acknowledgement Number: Contains the next sequence number that the
destination host wants to receive.
 Hdr Len: The length of the header in 32-bit words.
 Reserved: Reserved for future use.
 Flags: There are 6 bits for flags in the TCP header, each is used as follows.
 URG: If the first bit is set, an urgent message is being carried.
 ACK: If the second bit is set, the acknowledgement number is valid.

 PSH: If the third bit is set, it is a notification from the sender to the
receiver that the receiver should pass all the data received to the
application as soon as possible.
 RST: If the fourth bit is set, it signals a request to reset the TCP
connection.
 SYN: The fifth bit of the flag field of the packet is set when initiating a
connection.
 FIN: The sixth bit is set to terminate a connection.
 Window Size: The maximum number of bytes that a receiver can accept.
 TCP Checksum: Covers both the TCP header and TCP data.
 Urgent Pointer: This pointer is valid only if the URG flag is set.
B. UDP Header Format
If the protocol type in IP header has value 17, then the packet is a UDP
packet. UDP header has size 8 bytes. Figure (4.10) shows the UDP header that
comes after IP header.
Figure (4.10): UDP header

The UDP header fields are explained below:
 Source Port Number: The port number of the source process.
 Destination Port Number: The port number of the process running in the
destination host.
 Length: Length of UDP header and UDP data.
 Checksum: Checksum of both the UDP header and UDP data fields.
Figure (4.11): TCP segment and IP header
HW: Draw the figure of UDP segment and IP header?

Chapter Five: The Addressing of TCP/IP Protocols Computer Networks
Chapter Five
The Addressing of TCP/IP Protocols
Introduction
The identifier used in the IP layer of the TCP/IP protocol suite to identify each
device connected to the Internet is called the Internet address or IP address.
There are two versions of IP the TCP/IP protocol: IP version 4 and IP version 6.
IP version 6 is much more complicated than IP version 4 and is much newer. First, we
will be working with IP version 4 which is the address format of the four digits
separated by full-stops.
5.1. IPv4 Addresses
An IPv4 address is a 32-bit number, usually represented as a four-part decimal
number with each of the four parts separated by a period or decimal point, which
means that the address space is 232
or 4,294,967,296 (more than 4 billion). This
means that, theoretically, if there were no restrictions, more than 4 billion devices
could be connected to the Internet.
There are two prevalent notations to show an IPv4 address: Binary Notation
and Dotted Decimal Notation:
 Binary Notation: In binary notation, the IPv4 address is displayed as 32 bits.
Each octet is often referred to as a byte. So it is common to hear an IPv4
address referred to as a 32-bit address or a 4-byte address. The following is an
example of an IPv4 address in binary notation:
01110101 10010101 00011101 00000010

 Dotted-Decimal Notation: To make the IPv4 address more compact and easier
to read, Internet addresses are usually written in decimal form with a decimal
point (dot) separating the bytes. The following is the Dotted-Decimal notation
of the above address:
An IP address has two parts:
 Network
 Host (also known as local or node)
Each network has an Internet address. Each network also must know the
address of every other network with which it communicates.
After the network is identified, the specific host or node must be specified. A
unique host address for the particular network is added to the end of the IP address.
5.1.1 IPv4 addresses Classes
The address class determines which part of the address represents the network
bits (N) and which part represents the host bits (H). IP addresses are divided into
five classes:
 Class A: Large networks.
 Class B: Medium-sized networks.
 Class C: Small networks with less than 256 devices.
 Class D: Multicasting.
 Class E: Reserved.

Only Class A, B, and C addresses are used for addressing devices; Class D is
used for multicast groups, and Class E is reserved for experimental use. All
addresses are placed in a particular class based on the decimal values of their first
octets. In the first octet, an IP address can start with a decimal value between 1 and
255. The system of class addresses has been set up to help ensure assignment of
unique IP addresses. Only classes A, B, and C are available for commercial use.
Figure (5.1): IP Datagram Classes
Table (5.1): IP Address Classes A, B, C, D and E Are Available for Addressing Devices
Class Format Identifiers Range Network Bits
Networks
Available
Host bits Hosts Available
A N.H.H.H 0 1 to 126 8 [7 bits (first byte)] 126 (27
-2) 24 bits (last three bytes) 16,777,214 (224
-2)
B N.N.H.H 10 128 to 191 16 [14 bits (first two bytes)] 16,384 (214
) 16 bits (last two bytes) 65,534 (216
-2)
C N.N.N.H 110 192 to 223 24 [21 bits (first three bytes)] 2,079,152 (221
) 8 bits (last byte) 254 (28
-2)
D - 1110 224 to 239 Ranges from 224.0.0.0 to 239.255.255.255 → (268,435,456)
E - 1111 240 to 255 Reserved → (268,435.456)
All addresses in IPv4 → 4,294,967,296
Addresses in class A → 2,113,928,964
Addresses in class B → 1,073,709,056
Addresses in class C → 528,104,608

Class A
Class A was designed for very large networks only. Since only 1 byte in class A
defines the Netid and the leftmost bit should be 0, the next 7 bits can be changed to
find the number of blocks in this class. The class A range of network blocks will be
found:
00000000 = 0
01111111 = 127
Therefore, class A is divided into 126 (27
minus 2) blocks (because some
blocks were reserved as special blocks). However, each block in this class contains
16,777,214 (16,777,216 minus 2) addresses. Many addresses are wasted in this
class. Figure (5.2) shows the block in class A.
Figure (5.2): Blocks in class A
In a Class A network address, the first byte is assigned to the network address
and the three remaining bytes are used for the node addresses. The Class A format
is:
Network.Host.Host.Host
For example, in the IP address 49.22.102.70, the 49 is the network address,
and 22.102.70 is the host address. Every machine on this particular network would
have the distinctive network address of 49.

Class B
Class B was designed for medium-sized networks. Since 2 bytes in class B
define the class and the two leftmost bit should be 10 (fixed), the next 14 bits can
be changed to find the number of blocks in this class. The range of class B network
will be found:
10000000 = 128
10111111 = 191
Therefore, class B is divided into 16,384 (214
) blocks. However, each block in
this class contains 65,534 (65,534 minus 2) addresses. Many addresses are wasted
in this class. Figure (5.4) shows the blocks in class B.
Figure (5.4): Blocks in class B
In a class B network address, the first 2 bytes are assigned to the network
address, and the remaining 2 bytes are used for host addresses. The format is:
Network.Network.Host.Host
For example, in the IP address 172.16.30.56, the network address portion is
172.16, and the host address portion is 30.56.
Class C
Class C was designed for smaller networks. Since 3 bytes in class C define the
class and the three leftmost bits should be 110 (fixed), the next 21 bits can be
changed to find the number of blocks in this class. Here’s the range for a class C
network:
11000000 = 192
11011111 = 223

Therefore, class C is divided into 2,097,152 (221
) blocks, in which each block
contains 254 (256 minus 2) addresses. However, not so many organizations were
so small as to be satisfied with a class C block. Figure (5.5) shows the blocks in
class C.
Figure (5.5): Blocks in class C
The first 3 bytes of a class C network address are dedicated to the network
portion of the address, with only one measly byte remaining for the node address.
The format is:
Network.Network.Network.Host
Using the example IP address 192.168.100.102, the network address is
192.168.100, and the node address is 102.
Class D
Class D is the multicast address range and cannot be used for networks. There
is no network/host structure to these addresses. They are taken as a complete
address and used as destination addresses only, just like broadcast addresses.
Figure (5.6) shows the block.
Figure (5.6): Blocks in class D

The first 4 bits of a class D address must be 1110. The range for a class D
network:
11100000 = 224
11101111 = 239
Thus, class D multicast group addresses are from 224.0.0.0 to
239.255.255.255.
Class E
Class E is reserved for experimental purposes. There is just one block of class E
addresses. It was designed for use as reserved addresses, as shown in Figure (5.7).
Figure (5.7): Blocks in class E
The first 4 bits of a class E address must be 1111. The range for a class E
network:
11110000 = 240
11111111 = 255
Thus, class E ranged is from 240.0.0.0 to 255.255.255.255.
5.1.2 Classless Addressing
To overcome address depletion and give more organizations access to the
Internet, classless addressing was designed and implemented. In this scheme, there
are no classes, but the addresses are still granted in blocks.

Classless Inter-Domain Routing (CIDR) is an IP addressing scheme that was
developed after the class system of A, B, C, D, and E [uses a slash followed by a
number to highlight the network portion of an address instead of using a subnet
mask]. For example:
10.11.3.0/8 Class A
172.16.0.0/16 Class B
192.168.3.8/24 Class C
Other example:
192.168.3.15/26
172.21.165.1/19
The number after the slash is the number of bits that represent the network
portion of the IP address. CIDR was developed to increase the efficiency of
address allocation and to alleviate overloaded Internet routers.
5.2 IPv6 Addresses
IPv6 uses 128-bit or 16 byte addresses, which are exponentially larger than the
address size of IPv4. Therefore, IPv6 supports a number of addresses that is 4
billion times the 4 billion addresses of the IPv4 address space. This works out to
be:
IPv4 addresses (232
): 4,294,967,296
IPv6 addresses (2128
): 40,282,366,920,938,463,463,374,607,431,768,211,456
IPv6 addresses are written in hexadecimal form, it uses A, B, C, D, E, and F to
represent 10, 11, 12, 13, 14, and 15. The decimal 16 is represented in hexadecimal
as 10. The address below is an example of an IPv6 address:
EFDC: BA62:7654:3201: EFDC: BA72:7654:3210

Each section of hex characters represents 2 byte (or 16 bits) of the address. The
concept of class was never used in IPv6.
5.3. Hardware Address
Within every frame of data is a header that contains addressing information.
This header enables the packet to arrive at the correct location. This addressing
information comes from a physical address that is burned into every Network
Interface Card (NIC). NIC is a piece of hardware that is used to connect a host to a
network. When the card is manufactured, this address will not change for the life of
the card. This burned-in address can be called any of the following:
 Hardware address
 Media Access Control (MAC) address
 Ethernet address
 Physical address
 Network Interface Card (NIC) address
The hardware address is unique to all the network cards ever manufactured. It
is a 12-character hexadecimal address. A hardware address looks similar to this:
00:A0:C9:0F:92:A5
The first six of these hexadecimal characters represent the manufacturer and are
unique to the network card’s manufacturer. The last six characters form a unique
serial number that the card’s manufacturer has assigned to it.

5.4. Subnet Masks
A subnet mask is a 32-bit binary number that can be expressed in either
dotted-decimal or dotted-binary form. A subnet mask is allows the recipient of IP
packets to distinguish the network ID portion of the IP address from the host ID
portion of the IP address. The network administrator creates a 32 bit subnet mask
composed of 1s and 0s. The 1s in the subnet mask represent the positions that refer
to the network or subnet addresses.
Table (5.2) shows concept of a dotted-binary and dotted-decimal equivalents of
subnet masks for the various classes of IP addresses.
Table (5.2) Default Subnet Masks
Address
Class
Format
Dotted-Decimal
Form
Dotted-Binary Form
Class A N.H.H.H 255.0.0.0 11111111.00000000.00000000.00000000
Class B N.N.H.H 255.255.0.0 11111111.11111111.00000000.00000000
Class C N.N.N.H 255.255.255.0 11111111.11111111.11111111.00000000
Class A, B, and C addresses can be divided into smaller networks, called
subnetworks or subnets, resulting in a larger number of possible networks, each
with fewer host addresses available than the original network. The addresses used
for the subnets are created by borrowing bits from the host field and using them as
subnet bits. A subnet mask indicates which bits have been borrowed.
In other words, the router does not determine the network portion of the
address by looking at the value of the first octet; rather, it looks at the subnet mask
that is associated with the address. In this way, subnet masks let you extend the
usage of an IP address. This is one way of making an IP address a three-level
hierarchy, as shown in Figure (5.8).

Figure (5.8): A Subnet Mask Determines How an IP Address Is Interpreted
Table (5.3) shows the dotted-decimal and dotted-binary forms of subnet masks
that are permissible when it subnet a Class C address. The borrowed bits are
indicated in bold.
Table (5.3) Subnet Masks for C class
Borrowed
Bits
IP Address Dotted-Binary Form Subnet Mask
after subnetting
0 192.168.0.1/24 11111111.11111111.11111111.00000000 255.255.255.0
2 192.168.0.1/26 11111111.11111111.11111111.11000000 255.255.255.192
3 192.168.0.1/27 11111111.11111111.11111111.11100000 255.255.255.224
4 192.168.0.1/28
11111111.11111111.11111111.11110000
255.255.255.240
5 192.168.0.1/29
11111111.11111111.11111111.11111000
255.255.255.248
6 192.168.0.1/30
11111111.11111111.11111111.11111100
255.255.255.252

Example: What is the subnet mask of the IP: 128.138.243.0/26 and what is the
range of host?
Solution:
No. of Network bits: 11111111.11111111.11111111.11000000
Subnet mask: 255.255.255.192
No. of host: 1) 256-192=64
2) 26
=64
HW: What is the default subnet mask and subnet mask after subnetting of the IP ?
192.168.112.0/21
10.1.1.0/27

Computer Networks Introduction

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