In the light of development of networks, the concept of network was extended. Let us take a look at the common features to the current networks:

1- To make a network you need a transfer of data whether wire or wireless.

2- Those computers need adapter to connect computers by cables to make a network and those adoptions are called network interface card.

Computers which provide data or resources in the current network are called servers. Computers which benefit those data or resources are called client. In network, it is possible that one computer plays the role of server and client in the same time. For example, one computer on network may be server for printing and client for getting data from other server.

Network need to a program concerning to networks on the computer which is connected to network whether server or client. This program whether network operating system (NOS) or operating system includes a program to manage networks such as Windows NT. This program controls in the components of network and maintenance of connection between the client and server.

Firstly, networks consists of a few number of computers, may not exceed ten computers are connected together and with printer. This type of network is known as Local Area Network (LAN).

LAN usually uses one type from the means of connection and sometimes uses more than one type. Theses means are one of the following:

1- Twist pair Cable (Shielded or Unshielded).

2- Coaxial cable.

3- Fiber optic cable.

4- Wireless transmission media.

Now, let us take a look at Metropolitan Area Networks (MAN). It is other type in the classification of networks and based on the network technology. It is has a high speed and usually covers a wide area from 20 KM to 100 KM.