Classful vs Classless Routing Protocols

Classful routing protocols do not send subnet mask information with their

routing updates. A router running a classful routing protocol will react in one

of two ways when receiving a route:

• If the router has a directly connected interface belonging to the same

major network, it will apply the same subnet mask as that interface.

• If the router does not have any interfaces belonging to the same major

network, it will apply the classful subnet mask to the route.

Belonging to same “major network” simply indicates that they belong to the

same “classful” network. For example:

• 10.3.1.0 and 10.5.5.0 belong to the same major network (10.0.0.0)

• 10.1.4.5 and 11.1.4.4 do not belong to the same major network

• 192.168.1.1 and 192.168.1.254 belong to the same major network (192.168.1.0)

• 192.168.1.5 and 192.167.2.5 do not belong to the same major  network.

Take the following example (assume the routing protocol is classful):

If Router B sends a routing update to Router A, it will not include the subnet

mask for the 10.2.0.0 network. Thus, Router A must make a decision.

If Router A has a directly connected interface that belongs to the same major

network (10.0.0.0), it will use the subnet mask of that interface for the route.

For example, if Router A has an interface on the 10.4.0.0/16 network, it will

apply a subnet mask of /16 to the 10.2.0.0 network. If Router A does not have

 a directly connected interfacing belonging to the same major network, it will

apply  the classful subnet mask of /8. This can obviously cause routing difficulties.

When using classful routing protocols, the subnet mask must remain

consistent throughout your entire network.