IP Routing

Directly Connected Networks

When our Canadian visitor finally picks up her bags and is ready to head out of Grand Central Terminal, she looks around for the exit signs. On one end, below a row of immense windows, is a sign saying “Vanderbilt Avenue.” Below the opposite row of tall windows is a sign saying “Lexington Avenue.” Under the large stone arches is a sign reading “42nd Street” (Figure 1-1).

Figure 1-1. Grand Central Terminal and the adjoining streets

Just as the streets around Grand Central Terminal are immediately accessible to any traveler, a router has directly attached networks that are immediately accessible (in other words, that do not require any specific routing mechanism to discover). Consider router R, in the following example. Networks 1.0.0.0, 10.1.1.0, and 10.1.2.0 are directly connected to the router:

hostname R
!
interface Ethernet0
ip address 1.1.1.1 255.0.0.0
!
interface Ethernet1
ip address 10.1.1.4 255.255.255.0
!
interface Ethernet2
ip address 10.1.2.4 255.255.255.0
...

In fact, the moment these networks are connected to the router they are visible in R’s routing table. Note in the following output that the command to display the routing table is show ip route (in EXEC mode). Also note the “C” that is prepended to the entries in the routing table, indicating that the routes were discovered as directly connected to the router:

R#show ip route Codes: C - connected, ...