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A computer network is a group of computers connected to each other. Computer networks function in various ways and are referred to as personal area networks (PAN), local area networks (LAN), wide area networks (WAN), campus area networks (CAN) or metropolitan area networks (MAN).

A PAN is established over small areas when one device is connected to another over a few metres, for example the computer and the printer, the fax and the scanning machine or a higher level network. A LAN is a larger network covering larger areas like a building or across an office. A WAN covers a relatively large geographic area like one city to another and one country to another and often uses transmission facilities provided by common carriers, such as telephone lines. A CAN connects two or more LANs within a specific geographical area such as a college campus. A MAN connects two or more LANs or CANs together but within the boundaries of the immediate town, city, or metropolitan area. There is also the global area network (GAN) which is a network for mobile communications across an arbitrary number of wireless LANs or satellite coverage areas for example.

The other classification of networks is based on the hardware they use to connect such as optical fibre, Ethernet, wireless LAN, HomePNA, or power line communication. Ethernet is the standard interface for networks and uses physical wiring to connect devices. Wireless LAN technology, as the name indicates, is designed to connect devices without wiring and use radio frequencies for connection.

A network can be classified by its functional relationship between the nodes known as active networking, client–server or peer-to-peer. Network topology is another method of classification and signifies the way in which devices in the network see their logical relations to one another. Based on this logic, a network could be a bus, star, ring, mesh, tree or an hierarchical network. Another way of classifying a network is by the communications protocol it uses. For example, Internetworking, Intranet, Extranet and the Internet are popular forms of networking.

The Internet connects two or more distinct computer networks or network segments together to form an internetwork, shortened to Internet. An Intranet is a set of interconnected networks, using the Internet Protocol (IP). An Extranet is an internetwork that belongs to a single organisation but also has limited connections. Technically, an Extranet may also be categorized as a CAN, MAN, WAN, or other type of network, except that an Extranet cannot consist of a single LAN – it must have at least one connection with an external network.