Networking Router
The University of Findlay's College of Education

The router is a layer 3 device. Routers separate different network segments. A segment is a group of computers all with the same network ID (review subnetting for network ID definition). If one segment needs to talk to a different segment, the traffic will go through a router.

A router is a network traffic cop.

Computers one and two are on one segment and computers three and four are on another segment. One and two have the same network ID, so they can communicate without going through the router. In fact, the router will prevent traffic from a conversation between computers one and two from going out into other segments.

If computer one needs to communicate with computer four, the router with pass that traffic out to the router on the segment with computer three and four.

Usually the route IP address is the gateway address that is entered in the TCP/IP stack of the computer's operating system.

Unlike a switch or hub, a router is not a "generic" network device that just needs to be plugged into the network in order to work. A router is a "logical" component of the network. Before a router can work in a specific network it must be programmed. At a bare minimum, the router must know what IP addresses are connected to each interface of the router.