Classfull vs Classless Routing Protocols

Classless routing protocols do send the subnet mask with their updates. Thus, Variable Length Subnet Masks (VLSMs) are allowed when using classless routing protocols. Examples of classful routing protocols include RIPv1 and IGRP. Examples of classless routing protocols include RIPv2, EIGRP, OSPF, and IS-IS.

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.