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From: JBVB@AI.AI.MIT.EDU ("James B. VanBokkelen")
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Multiple subnets on one physical net
Message-ID: <281189.871107.JBVB@AI.AI.MIT.EDU>
Date: Sat, 7-Nov-87 12:33:35 EST
Article-I.D.: AI.281189.871107.JBVB
Posted: Sat Nov  7 12:33:35 1987
Date-Received: Sat, 14-Nov-87 15:50:50 EST
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As usual, everyone was right, in the context.  Mike Karels described
the behaviour of his Unix, 4.3, which ought to work right in this
situation.  Other people have described *their* Unices, and other O/Ss,
like Phil's LISPMs, which are going to give trouble.

There are several problems you'll run into:  The first is that some 4.2-
derived systems believe in broadcast addresses of Net.0, and other systems
believe in Net.255.  The second is that some manufacturers leave their
Unices believing that they are IP routers, even when they have only one
network interface.  The third is that some manufacturers don't include
code to implement the law "don't reply to broadcast packets unless you're
*really* *sure* you ought to".  The fourth is that some manufacturers
don't understand about subnets.

The end result of this is that on one net I know, an Old-Broadcastian sends
an rwho packet, and 29 (or more - my monitor was only an XT) Sun and Vax
New-Broadcastians immediately attempt to either forward the packet (they
think they're gateways), or send an ICMP Net Unreachable (gateway-ism
partly disabled).  Great fun.  Also available between systems that do
and don't grok subnets, and the two together are greater than the sum
or the parts.  Phil Karn cited some misbehavers.  There are others.  All
can be fixed if you have source, most can be fixed through patience and
vigilance on the part of a network administrator (watching and making
people correct mis-configured systems).

jbvb