The default network command, which has actually been removed in the very latest versions of IOS (or, perhaps, has been removed in versions not yet released on CCO), is used to mark a major network route as a "candidate default." Candidate defaults are considered, along with default routes (0/0), as possible gateways of last resort. The default network was originally created in IGRP because there's no way to encode the default route (0/0) in IGRP, actually, and was carried into EIGRP for backwards compatability, although EIGRP actually supports default routes (0/0) as well as candidate defaults.
The default network capability is being removed because IGRP has been removed from later versions of IOS, and newer versions of code also support new features that are not compatible with the default network (primarily in the area of MTR, to be specific).
HTH
:-)
Russ.W
