To route traffic from a source host to a destination host, the routers through which the traffic will pass must learn the path that the packet is to take. Once learned, the information is stored in routing tables. The routing table maintains a list of all the possible paths from point A to point B. Figure 45 shows a simple network of routers.
Figure 45: Simple Network Topology
This simple network provides multiple ways to get from Host San Francisco to Host Miami. The packet can follow the path through Denver and Cleveland. Alternatively, the packet can be routed through Phoenix and directly to Miami. The routing table includes all the possible paths and combinations—an exhaustive list of all the ways to get from the source to the destination.
The routing table must include every possible path from a source to a destination. Routing tables for the network in Figure 45 must include entries for San Francisco-Denver, San Francisco-Cleveland, San Francisco-Miami, Denver-Cleveland, and so on. As the number of sources and destinations increases, the routing table quickly becomes large. The unwieldy size of routing tables in the primary reason for the division of networks into subnetworks.