Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!snorkelwacker!bloom-beacon!bu.edu!buit13!kwe
From: kwe@buit13.bu.edu (Kent England)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Many logical nets on a single physical net
Summary: ARP for everything
Message-ID: <52813@bu.edu.bu.edu>
Date: 26 Feb 90 17:56:06 GMT
References: <1990Feb23.214634.8645@wang.com>
Sender: news@bu.edu.bu.edu
Reply-To: kwe@buit13.bu.edu (Kent England)
Followup-To: comp.protocols.tcp-ip
Organization: Boston U. Information Technology
Lines: 36

In article <1990Feb23.214634.8645@wang.com> fitz@wang.com
 (Tom Fitzgerald) writes:
>Is it possible to have a single Ethernet with multiple IP network addresses
>on it, and get the separate IP nets talking to each other?  Especially, is
>it possible to do this cheaply?
>

	You need to be able to tell your hosts to ARP for all
addresses.  In other words, you need to make them think that every
other host is reachable directly.  The net part is zero length; the
entire address is "local part".  This gets you what you want without
the extra hop of a single-interface gateway.  If you add a real
gateway somewhere at some future time, and if it supports proxy ARP
correctly, this will work thru a transition from bridged/repeatered
LANs to properly internetted LANs, until such time as you can come
back to all your hosts and set a new subnet/net mask and stop using
proxy ARP if you choose.

	However, since there is a "built-in" net mask defining
net/local for class A, B, C, etc, this "zero-length" network mask may
not work in specific products, depending on how subnet masks were
implemented.  In fact, it might not be RFC legal.  (Must check that
Host Requirements RFC again.)  I don't know of anyone offhand who is
using proxy ARP this aggressively.  Most people just use it to handle
backward compatibility for the subnet problem between 4.2 and 4.3 BSD.
So their hosts are still using the "built-in" class-based mask or are
using subnet masks as an "extension", rather than "replacement" of the
built-in mask.  (Maybe you can set the subnet mask to be a negative
number? :-)

	But this does solve the dynamic gateway discovery problem
rather neatly.  That's it!  Let's do away with net/local altogether
and have hosts ask for pathways to every IP address they want.  Does
that sound like an advanced end-system protocol?  :-)

	Kent England, Boston University