Classful Routing protocols do not send subnet mask information when a route update is sent out. All devices in the network must use the same subnet mask E.g.: RIP V1
Classless Routing is performed by protocols that send subnet mask information in the routing updates. Classless routing allows VLSM (Variable Length Subnet Masking) E.g.: RIP V2, EIGRP, & OSPF.
Below table clearly differentiates both Classful and Classless Routing –