While a router can be used to segment LANs, its major use is as a WAN device. Routers have both LAN and WAN interfaces. In fact, WAN technologies are frequently used to connect routers and these routers communicate with each other by WAN connections. Routers are the backbone devices of large intranets and of the Internet. They operate at Layer 3 of the OSI model, making decisions based on network addresses. The two main functions of a router are the selection of best path for and the switching of frames to the proper interface. Routers accomplish this by building routing tables and exchanging network information with other routers.

An administrator can maintain routing tables by configuring static routes, but generally routing tables are maintained dynamically through the use of a routing protocol that exchanges network topology (path) information with other routers.

If, for example, computer (x) needs to communicate with computer (y) on one side of the world, and with computer (z) in another distant location, a routing feature for information flow is required as well as redundant paths for reliability. Many network design decisions and technologies can be traced to this desire for computers x, y, and z to be able to communicate.

A correctly configured internetwork provides the following:

• Consistent end-to-end addressing
• Addresses that represent network topologies
• Best path selection
• Dynamic or static routing
• Switching

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