Using classful IP addresses meant that the subnet mask of a network address could be determined by the value of the first octet, or more accurately, the first three bits of the address. Routing protocols, such as RIPv1, only need to propagate the network address of known routes and do not need to include the subnet mask in the routing update. This is due to the router receiving the routing update determining the subnet mask simply by examining the value of the first octet in the network address, or by applying its ingress interface mask for subnetted routes. The subnet mask was directly related to the network address.
In Figure 1, R1 sends an update to R2. In the example, R1 knows that subnet 172.16.1.0 belongs to the same major classful network as the outgoing interface. Therefore, it sends a RIP update to R2 containing subnet 172.16.1.0. When R2 receives the update, it applies the receiving interface subnet mask (/24) to the update and adds 172.16.1.0 to the routing table.
In Figure 2, R2 sends an update to R3. When sending updates to R3, R2 summarizes subnets 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24 into the major classful network 172.16.0.0. Because R3 does not have any subnets that belong to 172.16.0.0, it applies the classful mask for a class B network, which is /16.