The default network command either marks an existing major network or creates and redistributes a major network into EIGRP as the "default network." This is a pseudo default network, which means that any traffic which doesn't have a matching destination in the routing table will be shipped to the router advertising the default network (just like a default route).
In newer CEF code, the default network is actually converted to a 0/0 route, to reclude special handling in the forwarding plane. If there is a default route (0/0) and a default network, this can lead to some unexpected results.
Historically, the default network command was put into the code to support IGRP--the way IGRP encodes metworks, there is no way to encode an empty network (0/0), thus IGRP couldn't support the default route, and a workaround had to be improvised. EIGRP supports this to provide backward compatibility with IGRP. No other routing protocol supports the default network--IE, a default network will cause a "gateway of last resort" to be added to the local routing table, but no protocol other than EIGRP (now that IGRP has been removed from the code) will advertise the route as a default network--it won't replace a default route in OSPF, IS-IS, or BGP.
Generally speaking, I don't recommend the use of the default network command at all. It's really far past it's usefulness, since the 0/0 route is supported by all current protocols, and it's confusing to troubleshoot and manage. Very few people expect the default route to be anything other than 0/0 any longer, so a lot of folks miss a default network even if it's in the table.
HTH
:-)
Russ
