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Any network ,whether big or small, is typically created so its users can share resources and communicate .There are many reasons for networking computers ,the most common being:
To share files: By networking computers ,it becomes very easy for the user on the network to share data files .
To share hardware: Users can share devices such as printers ,CD-ROM drives ,and hard drives for instance ,users can be assigned quotas of space on network server drives.

To share programs: Applications such as spreadsheets and word processors can be run over the network ,by keeping most of the files that make up that applications on a special networked application server.
Communication between users: Networks allow users to benefit from communication media such as e-mail ,newsgroups ,and video conferencing.
Multiplayer gaming: Users can be played together between a networking computers that to share in a long distance ,and have a support for multiple players on a network,
In the last years the number of computers is very increased; this fact is the result of people’s desire to be connected between each other.
The growth of connectivity would make the whole world closer to the users than in the past.
A computer network is a connection between many computers that could exchange information whit one another.
The networks could be differentiated in two ways:
-by topologies
-by extent

A topology is the way in which the different terminals (called nodes) could be linked.
The different kinds of topologies are basically three: star, ring, and bus, but a network could be produced by hybrid combinations of these topologies.
In the star topology there is a host computer connected to every other computer system: these smaller computer systems communicate whit other ones through the host and share the host computer’s database.
The disadvantage is that if the host computer is damaged, the whole system will be blocked.
The ring topology is the connection between computers of the same size: every computer is connected to the previous and the following.
A information pass, in the way of a message, computer by computer all over the circle until it reaches its destination.
The disadvantage is that there is a fault in a part of the ring, all the nodes will be blocked.
In bus topologies every node is connected to a common cable: a message is spread to all computer systems but only the destination node respond to the signal.
The advantage is that if a node is damaged, the others won’t be blocked.
In terms of scope, networks can be classified as follows:
Personal Area Network (PAN), wired and wireless: a small network that typically covers the few metres surrounding a user's workspace and provides the ability to synchronize computers, transfer files and gain access to local peripherals like printers and pocket devices. The connection types used for wired PANs are USB or FireWire, while Bluetooth or IrDA (infrared) connections may be used for wireless PANs.
Local Area Network (LAN), wired and wireless: a network that spans a relatively small area, usually a single building or group of buildings.
Metropolitan Area Network (MAN), wired and wireless: a regional computer or communication network spanning the area covered by an average sized to large city.
Wide Area Network (WAN): a large network spanning a country or the world, like the
Storage Area Network (SAN): a high-speed sub-network only made of shared storage devices, available to all servers of a LAN or WAN. Because stored data does not recide directly on any of
A network’s servers ,the server power can be used for applications and network capacity is released to the end user.