Path: utzoo!mnetor!uunet!husc6!bbn!uwmcsd1!ig!jade!ucbvax!ATHENA.MIT.EDU!mar
From: mar@ATHENA.MIT.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: More than one IP (sub)network on one ethernet cable
Message-ID: <8712211726.AA04950@TOTO.MIT.EDU>
Date: 21 Dec 87 17:26:54 GMT
References: <8712190319.AA26455@uc.msc.umn.edu>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 19

But there is a simpler solution to the one you described, and it is
even documented in RFC 1027.  This is ARP subnet routing, or the ARP
hack, as it is sometimes called.

This means having your gateway respond to arp requests for machines on
other subnets, giving it's own ethernet address.  For instance, if you
have a machine on subnet 1 of your class B network which does not
understand subnet routing, and this machine wants to send a packet to
a host on subnet 2, not knowing about subnets it would assume that it
can send directly and arp for the destination host's IP address.  The
gateway on subnet 1 receives this ARP, and responds with it's own
hardware address so that the packet gets sent to the gateway, which
can forward it to the correct destination.  So the ARP hack works in
the case of smart gateways and dumb hosts.

Proteon gateways support this, and I think Cisco ones do too.  There
are mods available for the 4.2 kernel, and it is already in 4.3, if
you enable it.
					-Mark