Internet Core Protocols: The Definitive Guide

Misconfigured subnet masks

Another common cause for high numbers of ICMP Redirect error messages can be seen on networks that have hosts with invalid subnet masks. Whenever these hosts try to connect to another system on the local network, they may think that the destination system is actually on a different network (due to the boundaries of the subnet mask in use), and as such will send the datagrams to the local router for delivery. The router would then inform the sender that the destination system was indeed local to the sender (using a Redirect error message for the job).

The best solution to this problem is to assign the correct subnet mask to the devices on the network, allowing them to communicate with each directly without one of them trying to contact a router first.