A router simply forwards packets of information between networks that use the same protocols. In a packet-switched network each packet of information contains the network address of its destination [6,3]. The router monitors all network traffic and forwards all packets that are addressed for machines not on the local network. The most common use of a router is in building a hierarchical network. A local network within an academic department will have a router that connects to the campus-wide network, which in turn will have a router or gateway to a regional network. Routers are also used to improve performance within a local network. If network traffic is too high, the network administrator might split it into two separate networks connected by a router. Routers are invisible to users, so in most situations sending information to a machine elsewhere in the department or on campus is no different than sending information to the machine in the lab next door.
The remainder of this section is an overview of the types of services typically provided by a local area network and how they impact the way you work. The examples will typically be from networks of Unix systems because Unix is so predominant in university and government labs, but the main concepts also apply to most other types of local area networks.