When classful protocols were originally developed, networks were very different from those used now. The best modem speed was 300 bps, the largest WAN line was 56 kbps, router memory was under 640KB, and processors were running in the kHz range. Routing updates had to be small enough not to monopolize the WAN link bandwidth. In addition, routers did not have the resources to maintain up-to-date information about every subnet.
A classful routing protocol does not include subnet mask information in its routing updates. Because no subnet mask information is know, when a classful router sends or receives routing updates, the router makes assumptions about the subnet mask being used by the networks listed in the update. These assumptions are based on IP address class. Upon receiving a routing update packet, a router running a classful routing protocol does one of the following to determine the network portion of the router:
All subnets of the same major network, Classes A,B and C, must use the same subnet mask when using a classful routing protocol. Otherwise, routers may assume incorrect subnet information. Routers running a classful routing protocol perform automatic route summarization across network boundaries.