A network is a number of computers and their peripheral devices connected together in some way. 

• Each device in a network is called a node. 
• The nodes in a network include many different types of devices, such as printers, storage devices, terminals and workstations. 
• Terminals are devices that send data to and receive data from another computer system. 
• If the terminal has both memory and processing capabilities, it is called an intelligent terminal. 
• Most personal computers are classified as intelligent terminals and are called workstations on a network.

The simplest form of a network is when one computer is connected directly to another computer, using a cable.However, a network can also consist of thousands of computers connected together. Networks are classified as local area networks or wide area networks:

• Local area networks (LANs) 
• connect computers within a building or group of buildings on one site. 
• LANs cover a small geographical area, and the computers are linked together by coaxial cable or fibre-optic cable. 
• There are three main advantages in using a LAN:
• – sharing limited hardware resources, such as printers, hard disks and modems 
• – sharing application software, such as word processing, database, spreadsheet and graphics programs
• – improved communication among users by sending electronic messages.

• Wide area networks (WANs) connect computers over hundreds or thousands of kilometres.
• WANs often consist of a mainframe computer called the host and a number of terminals. 
• For example, the EFTPOS terminals of many retail organisations and the ATM terminals of banks are all part of a WAN (see Figure 3.11). 
• A WAN may use a private leased line, the normal telephone network or a combination of both. 
• A private leased line is dedicated to the network. 
• It offers higher transmission speeds and more accurate transfer of data than those available through the public switched telephone network (PSTN). 
• PSTN links are relatively cheap if computers do not require constant connection. 
• However, PSTN links are not guaranteed and can be interrupted.

Most data sent over a network uses packet switching. 

• Packet switching is a technique that divides messages into small data packets, transmits the packets and later joins the packets to form the original message. 
• It allows multiple users to use the same transmission line by interspersing the data packets from different users. 
• Data packets may not be sent along the same path and could arrive at the destination at different times and in the wrong order. 
• Each data packet contains an address and control instruction to reassemble the message in the correct order.
• Data transfer on the Internet is based on packet switching.