Path: utzoo!mnetor!uunet!husc6!hao!ames!ucbcad!ucbvax!UMN-REI-UC.ARPA!slevy
From: slevy@UMN-REI-UC.ARPA (Stuart Levy)
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  More than one IP (sub)network on one ethernet cable
Message-ID: <8712211826.AA10567@uc.msc.umn.edu>
Date: 21 Dec 87 18:26:29 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 16

We're already doing subnet (proxy) ARP -- not exactly with our gateways,
which don't have this feature built in (they're things like SUNs)
but using a program employing the SUN `nit' ethernet-tap interface
to listen for ARPs and respond from a table, giving the Ethernet address
of the gateway, just as you suggest.

This works fine for really dumb hosts that aren't using subnetting,
and gives us "transparent" subnets.

The case where it -doesn't- work is for hosts that -are- using subnetting,
e.g. for the subnet gateways themselves.  Say we want to give a default
route (to 128.101.1.3) to one of them on a subnet other than 128.101.1.*.
The UNIX routing software won't permit this -- it demands that gateways
be on the same (sub)net as some interface address.  So we synthesize
these fake gateways (actually by building them into the same table that
handles subnet ARPs) so that every subnet has a fake gateway within reach.