A network of some sort is required in order for computers to communicate. As already discussed from a hardware point of view in Chapter 2, Workstation Setup and Use, local area networks (LANs) connect machines in a department or building. Most often the LAN is an ethernet. Networks can connect more than Unix workstations and often do connect everything from PCs to supercomputers, yet in order to attain the single-image level of transparency described above with present technology, all of the machines must run some variant of Unix.
One of the nicer things about local networks is that they are not isolated, being interconnected to yet other networks by special-purpose computers called routers and bridges. The purpose of a router is to keep local traffic confined to the local network and to provide a controlled route for traffic between the local network and other networks and their machines. Routers often connect departmental networks to a sitewide or campuswide backbone. A backbone is a special purpose LAN which connects routers rather than computers and thus indirectly interconnects the networks. Backbones are not confined to campus systems. They may be regional or even international.