Classful route lookups can be illustrated with three examples (referring to Figure 5.5):
1. If a packet with a destination address of 192.168.35.3 enters this router, no match for network
192.168.35.0 is found in the routing table and the packet is dropped.
2. If a packet with a destination address of 172.25.33.89 enters the router, a match is made to class B
network 172.25.0.0/24. The subnets listed for this network are then examined; no match can be
made for subnet 172.25.33.0, so that packet, too, is dropped.
3. Finally, a packet destined for 172.25.153.220 enters the router. This time 172.25.0.0/24 is
matched, then subnet 172.25.153.0 is matched, and the packet is forwarded to next-hop address
172.25.15.2.
Another look at Figure 5.3 reveals that there is no provision for RIP to advertise a subnet mask along with
each route entry. And accordingly, no masks are associated with the individual subnets in the routing
table. Therefore, if the router whose forwarding database is depicted in Figure 5.5 receives a packet with
a destination address of 172.25.131.23, there is no positive way to determine where the subnet bits end
and the host bits begin, or even if the address is subnetted at all.
The router's only recourse is to assume that the mask configured on one of its interfaces attached to
172.25.0.0 is used consistently throughout the internetwork. It will use its own mask for 172.25.0.0 to
derive the subnet of the destination address. As the routing tables throughout this chapter illustrate, a
router that is directly connected to a network will list the network in a heading along with the subnet mask
of the connecting interface and will then list all the known subnets of the network. If the network is not
directly connected, there is a listing only for the major-class network and no associated mask.
Because the destination addresses of packets being routed by a classful routing protocol are interpreted
according to the subnet masks locally configured on the router's interfaces, all subnet masks within a
major, class-level network must be consistent.