The biggest distinction between classful and classless routing protocols is that classful routing protocols do not send subnet mask information in their routing updates. Classless routing protocols include subnet mask information in the routing updates.
The two original IPv4 routing protocols developed were RIPv1 and IGRP. They were created when network addresses were allocated based on classes (i.e., class A, B, or C). At that time, a routing protocol did not need to include the subnet mask in the routing update, because the network mask could be determined based on the first octet of the network address.
Note: Only RIPv1 and IGRP are classful. All other IPv4 and IPv6 routing protocols are classless. Classful addressing has never been a part of IPv6.
The fact that RIPv1 and IGRP do not include subnet mask information in their updates means that they cannot provide variable-length subnet masks (VLSMs) and classless interdomain routing (CIDR).
Classful routing protocols also create problems in discontiguous networks. A discontiguous network is when subnets from the same classful major network address are separated by a different classful network address.
To illustrate the shortcoming of classful routing, refer to the topology in the Figure 1. Notice that the LANs of R1 (172.16.1.0/24) and R3 (172.16.2.0/24) are both subnets of the same class B network (172.16.0.0/16). They are separated by different classful network addresses (192.168.1.0/30 and 192.168.2.0/30).
When R1 forwards an update to R2, RIPv1 does not include the subnet mask information with the update; it only forwards the class B network address 172.16.0.0.
R2 receives and processes the update. It then creates and adds an entry for the class B 172.16.0.0/16 network in the routing table, as shown in Figure 2.
Figure 3 shows that when R3 forwards an update to R2, it also does not include the subnet mask information and therefore only forwards the classful network address 172.16.0.0.
In Figure 4, R2 receives and processes the update and adds another entry for the classful network address 172.16.0.0/16 to its routing table. When there are two entries with identical metrics in the routing table, the router shares the load of the traffic equally among the two links. This is known as load balancing.
As shown in Figure 5, this has a negative effect on a discontiguous network. Notice the erratic behavior of the ping and traceroute commands.