Painting the Road Signs on Your Interstate (Internetwork)

If you take a trip in your car and you don't use a map, you can still get where you're going by just reading the road signs. Routing, as covered in Chapter 11, "Knowing Where to Turn at Each Intersection (Router)," works a lot like taking a trip and relying on road signs. Think of the IP packets as cars, each intersection as a router, and the routers' routing tables as the road signs. Just like a driver might rely on good information found on the road signs, IP packets rely on the routers' routing tables having good information in them. When a router receives a packet, it must match the destination IP address of the packet to the routing table to figure out where to send the packet next. Similar to cases in which you might see a road sign when driving and turn off onto another road, a router directs the packet down the next network roadway to get to its destination. (Note that the router forwards the packet and makes the decision of where to send the packet, which is a slight departure from the analogy with driving, in which the driver of the car decides where to turn.)

In short, IP packets rely on the routers having good, complete routing information in their routing tables. This brief chapter covers the most basic concepts of how a router creates and fills its routing table.