There are some cases in IP routing that can only be sensibly handled by
subinterfaces - they come up principally when you're talking to a switched
media of some kind with multiple IP network numbers in use (e.g. SMDS,
=46rame Relay, ATM). It's very important for the box (whatever it is, a
NetBSD, or a router or whatever) to respond to any packet with the correct
source address, and this is easier to deal with by using subinterfaces (and
selection of the source address for the generated packet from the routing
table) than forcing the applications to have to deal with that.

=46or the case where a host has multiple IP addresses on the same IP network
number (and, perforce, the same wire), the "alias" stuff is adequate. Life
gets more complicated when you have multiple IP network numbers in use on
what appears to be the same media.

	been there, done that,

	Erik Fair <fair@clock.org>