Routing to Nearby Places

Routing to Nearby Places

Imagine a sleepy little town with a couple of fellows sitting around at the only gas station. Because I grew up in a town like that, I'll call the town Snellville, after my hometown. These guys are just talking, waiting around for the next customer.

A stranger drives up, rolls down his window, and asks, "Excuse me. Can you tell me how to get to Snellville?" I'm sure a dozen funny or sarcastic answers would probably leap to mind, but the fellows at the gas station would eventually tell the stranger that he had just missed the sign that told him he was already in Snellville. No need to drive any further!

Interestingly, routers first fill their routing tables based on a similar concept. Each router knows which of its physical interfaces are up and working. It knows the IP addresses used on each interface. Each router also knows what IP networks or subnets exist on the physical networks that are connected to those interfaces. The router can add a route to the subnet that exists on the physical networks to which it is attached.

Before a router can add routes to these subnets, it must have an IP address assigned to each network interface. When you buy a brand new router, it doesn't know which IP addresses you want it to use. A network engineer needs to somehow tell the router which IP addresses to use; to do so, the engineer configures the router.

Configuring a router means that the engineer connects to the router and types in some information about what the router should do. For instance, in Figure 12-1, R1 needs to know its IP addresses for interfaces Ethernet1 and Ethernet2. When the engineer configures the IP address for each interface, he also (coincidentally) tells the router which subnets or networks are attached to those two interfaces. Figure 12-1 shows the basic concepts.