2.4 Variable-Length Subnet Masks
2.4.2 Classless and classful updates
VLSMs can be used when the routing protocol sends a subnet mask along with each network address. The protocols that support subnet mask information include RIP2, OSPF, Enhanced IGRP, BGP, and IS-IS. Networks running these protocols are called classless networks because they are not constrained by the Class A, B, and C designations that indicate the boundary for network and host portions. A prefix identifies the number of bits used for the network portion. This prefix accompanies all routing exchanges.

RIP1 and IGRP do not support VLSMs. RIP1 and IGRP networks support only one subnet mask per network address because routing updates do not have a subnet mask field. As a result, upon receiving a packet, the router does one of the following to determine the network portion of the destination address:

  • If the routing update information about the same network number is configured on the receiving interface, the router applies the subnet mask that is configured on the receiving interface.
  • If the router receives information about a network address that is not the same as the one configured on the receiving interface, it will apply the default (by class) subnet mask, which is why RIP1 networks are referred to as classful networks. RIP1 route updates do not have a subnet mask field.

In Figure to the left, for example, in the RIP network router B is attached to network 172.5.1.0/24. Therefore, if router B learns about any network that is also a subnet of the 172.5.0.0 network, it will apply the subnet mask configured on its receiving interface (/24).

But notice how router C, which is attached to the 192.168.5.0/24 subnet, handles network address 172.5.2.0. Rather than assigning the network address the correct subnet mask (/24), it applies the default (classful) subnet mask for a Class B network address --- 172.5.0.0.

It is impossible in this kind of environment to further subnet already subnetted IP addresses without causing confusion. Instead, VLSMs can be used only when the routing protocol sends subnet mask information along with the network address.