The main difference between a classful and classless protocol is, that classful protocols do not send subnet mask information in their router updates. Classful routing protocols also create problems in discontiguous networks. A discontiguous network means a subnet from the same class are seperated from the major network address. 

RIPv2 and IGRP are the original developed IPv4 protocols. They were in use back then when networks were allocated based on their classes. There was also no need for exchanging the subnet addresses since that could be determined based on the first octet of the addresses.

IPv4 and IGRP are the only classful protocols. IPv6 does not even have a classful protocol.

Figure 1

If an update ist send from R1 to R2, then RIPv1 does not send the subnet mask. It only sends the IP Address 172.16.0.0.

R2 recieves the updates and registers the IP address to the routing table. 

Figure 2

If R3 sends an update to R2 then it also does not include the subnet mask. The IP will be proceeded and registered.

Figure 3

R2 recieves an update and also registers it to the table. If there are 2 entries with identical metrics then the router shares the load of the traffic equally among the two links.

Figure 4

This has a negative effect on a discontiguous network. It causes an erratic behavior of the ping and traceroute commands.