Hi,
Ok, we must differentiate here between classful/classless routing protocols, and classful/classless router forwarding behavior.
The whole idea here is that Classful routing protocols don't advertise the subnet mask with the route, accordingly the router has to assume one when it receives the route via a classful routing protocol (and the router that has advertised the routes must do some checkups before sending it).
Kindly check the following document for the behavior of classful routing protocols
Behavior of RIP and IGRP When Sending and Receiving Updates
http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a0080093fd8.shtml
On the other hand, Classful route lookups, in which a destination address is first matched to its major network address in the routing table and is then matched to a subnet of the major network. If no match is found at either of these steps, the packet is dropped (A default route will only work if the major classful route is not in the routing table). For earlier IOS versions you can enable classless route lookup, even for classful routing protocols such as RIPv1 and IGRP, by entering the global command ip classless. When a router performs classless route lookups, it does not pay attention to the class of the destination address. Instead, it performs a bit-by-bit best match between the destination address and all its known routes.
HTH,
Mohammed Mahmoud.
