Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 99
IP Routing Tables

The slide shows an example of what is in a routing table. This is a distance-vector routing table, not a link-state routing table. OSPF would have a different kind of table. In this table, it is important to note the network number, hops to that network number, and the next router in the path to that network.

Routing table fields vary, depending on the update mechanism used. The following table is a sample of a routing table used by the routing information protocol (RIP) for the IP protocol.

The route table entries on the slide are defined as follows:

Network number. A known network ID.

Next router to deliver to. The next router that the packet should be delivered to if the destination network is not directly connected. A directly connected network is one that is physically connected to the router, since most routers today have more than two connected networks.

Hops. This is the metric count of how many routers the packet must traverse before reaching the final destination. A 1 indicates a local route.

Learned from. Since many routing algorithms may exist in a router (i.e., RIP, OSPF, and EGP may exist in the same router), there is usually an entry in the table to explain how the route was acquired.

Time left to delete. The amount of time left before the route will be deleted from the table.

Port. The physical port on the router from which the router received information about this network.

IP Routing Tables