Hello Seth,

I suggest you first try to read this topic that has been discussed here before:

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=WAN%2C%20Routing%20and%20Switching&topicID=.ee71a06&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40^1%40%40.2cd432f0/7#selected_message

You will need to copy the entire URL in clipboard as a text and paste it into your browser because the hyperlink automatically created here does not contain the entire URL, unfortunately.

In short, the "ip default-network" is a command that flags some network as a so-called candidate default route. This network is not supposed to be directly connected, rather it should be a network which is already "out there somewhere" and you already have it in your routing table by whatever means. Now, the next hop to this network also becomes the default next hop, as if you specified it in a normal default route. It is as if you said "the path to this particular network is the same as the default route".

This command was originally created for IGRP routing protocol and its behavior is outspokenly cumbersome, as you will read in the suggested thread I've quoted here. It is not accepted by OSPF or IS-IS. All in all, it is one of the most confusing elementary commands in IOS I know about.

Have a look at the linked thread and please come back with any questions you might have.

Best regards,

Peter