Classful routing protocols make assumptions about networks based on their IP address class. These assumptions lead to automatic summarization of routes when routers send updates across major classful network boundaries.
Routers send update packets from their interfaces to other connected routers. The router sends the entire subnet address when an update packet involves a subnet of the same classful network as the IP address of the transmitting interface. The router assumes that the network and the interface use the same subnet mask.
The router that receives the update also makes the same assumption. If that route was using a different subnet mask, then the router would have incorrect information in its routing table. Thus, when using a classful routing protocol, it is important to use the same subnet mask on all interfaces belonging to the same classful network.
When a router using a classful routing protocol sends an update regarding a subnet of a classful network across an interface belongging to a different classful network, the router assumes thatthe remote router will use the default subnet mask for that class of IP address. Therefore, when the router sends the update, it does not include the subnet information. The update packet contains only the classful network information. This process is autosummarization across the network boundary. The router sends a summary of all the subnets in that network by sending only the major network information. Classful routing protocols automatically create a classful summary route at major network information. Classful routing protocols automatically create a classful summary route at major network boundaries. Classful routing protocols do not allow summarization at other points within the major network address space.
The router that receives the update behaves in a similar fashion. When an update contains information about a different classful network than the one in use on its interface, the router applies the default classful mask to that update. The router must determine the correct subnet mask to apply because the update does not contain subnet mask information.