Wide area networks

A wide area network or WAN, spans a large geographical area, often a country or continent. We will call this machines host. The hosts are connected by a communication subnet, the job of the subnet is to carry messages from host to host, and just as the telephone system carries words from speaker to listener. The subnet consists of two distinct components: Transmission lines and Switching elements.

Transmission lines move bits between machines. The switching elements are specialized computers used to connect two or more transmission lines. They are called routers. The collection of communication lines and routers form the subnet.