A network connects two or more computer systems together to share data and information. There are different types of networks categorized by their geographic reach, such as local-area networks (LANs) for computers close together, and wide-area networks (WANs) for computers farther apart connected by telephone or radio. Networks are also classified by their topology like bus, star, ring, mesh, and tree, which define how the connected devices are arranged. Common network devices include hubs, switches, routers, and bridges that help direct data traffic flow across the network.

1. Networks
Network: A network is a group of two or more computer systems connected
together in order to exchange data and information.
Types of Networks:-
1. local-area networks (LANs): The computers are geographically close
together (that is, in the same building).
2. wide-area networks (WANs): The computers are farther apart and are
connected by telephone lines or radio waves.
3. campus-area networks (CANs): The computers are within a limited
geographic area, such as a campus or military base.
4. metropolitan-area networks MANs): A data network designed for a
town or city.
5. home-area networks (HANs): A network contained within a user's
home that connects a person's digital devices.
The following characteristics are also used to categorize different types of
networks:
 topology: The geometric arrangement of a computer system. Common
topologies include a bus, star, mesh and ring.
 protocol : The protocoldefines a common set of rules and signals that
computers on the network use to communicate. One of the most popular
protocols for LANs is called Ethernet. Another popular LAN protocolfor
PCs is the IBM token-ring network .
 architecture : Networks can be broadly classified as using either a peer-
to-peer or client/server architecture.
Network Topologies:-
1. Bus topologyis a network type in which every computer and network
device is connected to single cable. When it has exactly two endpoints,
then it is called Linear Bus topology. It transmits data only in one
direction. It is costeffective and used in small networks. But network
fails if cable is faulty.
2. Ring topologybecause it forms a ring as each computer is connected to
another computer, with the last one connected to the first. Exactly two
neighbours for each device. A number of repeaters are used for Ring
topology with large number of nodes (which have tokens) to prevent
loss of data. Data is transferred in a sequential manner that is bit by bit.
Data transmitted, has to pass through each nodeof the network, till the
destination node.

2. 3. In star topology, all the computers are connected to a single hub
through a cable. This hub is the central node and all others nodes are
connected to the central node. Every node has its own dedicated
connection to the hub. Hub acts as a repeater for data flow.
4. Meshtopologyis a point-to-point connection to other nodes or devices.
All the network nodes are connected to each other. Mesh has n(n-1)/2
physical channels to link n devices. Data is transmitted by routing or
flooding. PartialMesh Topology:In this topology, some of the
systems are connected in the same fashion as mesh topology but some
devices are only connected to two or three devices. Full Mesh
Topology:Each and every nodes or devices are connected to each
other.
5. Tree topologyhas a rootnode and all other nodes are connected to it
forming a hierarchy. It is also called hierarchical topology. It should at
least have three levels to the hierarchy.
6. Hybrid topologywhich can be a combination of two basic topologies.

3. Network devices:-
1. Hub: Network Hub is a broadcasting device which is used to connect
multiple network hosts. A network hub is also used to do data transfer.
The data is transferred in terms of packets on a computer network. So,
when a host sends a data packet to a network hub, the hub copies the data
packet to all of its ports connected to. Like this, all the ports know about
the data and the port for whom the packet is intended, claims the packet.
However, because of its working mechanism, a hub is not so secure and
safe. Moreover, copying the data packets on all the interfaces or
ports makes it slower and more congested.
2. Switch: While hub just does the work of data forwarding, a switch does
‘filter and forwarding’ which is a more intelligent way of dealing with the
data packets. So, when a packet is received at one of the interfaces of the
switch, it filters the packet and sends only to the interface of the intended
receiver. It has its own configuration and memory.
3. Modem: A modem stands for (Modulator+Demodulator). That means it
modulates and demodulates the signal between the digital data of a
computer and the analogue signal of a telephone line.
4. Router: A router is a network device which is responsible for routing
traffic from one to another network. These two networks could be a
private company network to a public network. You can think of a router
as a traffic police who directs different network traffic to different
directions.
5. Bridge: If a router connects two different types of networks, then a
bridge connects two subnetworks as a part of the same network; i.e.; it
divides a larger network into segments for more efficient transfer of data
and information.
6. Repeater:A repeater is an electronic device that amplifies the signal it
receives.
3 Tier Client-ServerArchitecture:-
1. Client which sends in the request to receive data from servers.
2. Application server (also called middleware), whose task it is to provide
the requested resources, but by calling on another server.
3. Data server, which provides the application server with the data it
requires.

4. Web servers are computers that deliver (serves up) Web pages. Every Web
server has an IP address and possibly a domain name.
OSI Reference Model:-
OSI (Open Systems Interconnection) is reference model or conceptual
framework for how applications can communicate over a network.
1. Physicallayer: This layer conveys the bit stream through the network at
the electrical, optical or radio level. It provides the hardware means of
sending and receiving data on a carrier network.
2. Data-link layer: This layer sets up links across the physical network,
putting packets into network frames. This layer has two sub-layers, the
Logical Link ControlLayer(manages traffic and protocol) and the Media
Access ControlLayer(shares data). Ethernet is the main data link layer in
use.
3. Network layer:This layer handles the addressing and routing of the data
(sending it in the right direction to the right destination on outgoing
transmissions and receiving incoming transmissions at the packet level).
IP is the network layer for the Internet.
4. Transport layer: This layer manages packetization of data, then the
delivery of the packets, including checking for errors in the data once it
arrives. On the Internet, TCP and UDP provide these services for most
applications as well.
5. Sessionlayer:Coordination, termination, authentication and
reconnection after termination of network connectivity is looked after by
session layer. Records time of session.

5. 6. Presentationlayer: This layer converts incoming and outgoing data
from one presentation format to another(encryption and decryption).
7. Application layer: It is the set of services an application should be able
to make use of directly. It is associated with identification and capacity
assessment of network.