Every IP routing protocol is classfull. ie, RIP, IGRP, EIGRP, OSPF, RIP V2.
It's the ability to route class A, B and C addresses.
It's when you you get into classless routing that some protocols don't support.
Alot of information to explain it all, so <A HREF="http://encyclopedia.thefreedictionary.com/classful network" target="_new">here</A> is some light reading.
Classful routing protocols do not include the mask in routing updates. For example, a RIP router will only send the IP of the network so that 10.5.2.0/24 and 10.1.0.0/16 will only be advertised as the 10.0.0.0/8 network. Classless routing protocols such as OSPF will include the mask and be able to see that these are different networks.
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