At 5:58 8/18/95, Perry E. Metzger wrote:
>I don't believe that you can set two different netmasks on the ip
>addresses on the interface, though I'll admit that I haven't
>checked. It would seem like a strange thing to want to do, though,
>because they both are on the same IP network and thus we would expect
>them both to use the same netmask.

You can on a cisco router, and we use this feature to good effect at Apple
- we have an A 17.0.0.0 subnet mask 255.255.252.0 (yes, 10 bits of host),
and a B 130.43.0.0 subnet mask 255.255.255.0 (much more normal). Most
commonly, this is done on a link or backbone network (with nothing but
cisco routers on it), to get both numbers to a terminal router, which then
splits things out to several different interfaces, to segregate an
application. We've also used this to make renumbering a network much
easier.

The primary catch is that all hosts on a network with multiple network
numbers (remembering that a router is a host) so configured really should
be aware of all network numbers in use on the network, and the masks.
Otherwise, at minimum, packets will take an extra hop from the hosts that
don't know to the ones that  do, and the hosts that don't know are likely
to be quite confused by any routing packets they hear.

Erik