Discontiguous Classful Networks

Discontiguous classful networks are networks that have different classful network addresses. In classful routing protocols such as RIPv1 and IGRP, the routers assume that all networks within a classful network have the same subnet mask, and they automatically summarize the routes at the classful network boundary. However, when there are discontiguous classful networks, autosummarization can cause routing problems, because the routers may incorrectly summarize routes or drop packets that belong to subnets that are not part of the summarized route.

For example, suppose we have two networks:

Network A: 192.168.1.0/24

Network B: 172.16.2.0/24

In these networks, the classful network boundaries are:

Network A: 192.168.0.0/16

Network B: 172.16.0.0/16 If we use a classful routing protocol such as RIPv1 or IGRP, the routers will automatically summarize these two networks into a single route, 192.0.0.0/8, because this is the classful network that includes both networks. However, this summarization is incorrect, because the two networks are not actually part of the same classful network.