Classful Routing: Summarization at Boundary Routers
The solution is simple: If a router has no direct attachments to the network, then it needs only a single route entry pointing toward a router that is directly attached.
NOTE
Boundary routers perform route summarization, also known as subnet hiding. Figure 5.6 shows a router that is attached at the boundary of two major networks, the class A network 10.0.0.0 and the class C network 192.168.115.0. This boundary router does not send details of the subnets of one major network into the other major network. As the illustration shows, it automatically performs summarization, or subnet hiding. It advertises only the address 10.0.0.0 into network 192.168.115.0 and advertises only the address 192.168.115.0 into network 10.0.0.0.
Figure 5.6. This router, at the boundary of two major networks, does not advertise the subnets of one network to routers within the other network.
In this way, the routing tables for routers within network 192.168.115.0 have only a single entry that directs packets for 10.0.0.0 toward the boundary router. The boundary router has an interface directly on network 10.0.0.0 and therefore has a subnet mask with which to derive the subnet for routing a packet within that network's "cloud." Figure 5.7 shows what the routing table of a router within 192.168.115.0 would look like with a single, subnetless entry for 10.0.0.0.
Figure 5.7. This router has a single entry pointing toward network 10.0.0.0. The next-hop address is the boundary router, since the network is recorded as being one hop away.
