Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!wuarchive!decwrl!ucbvax!JESSICA.STANFORD.EDU!almquist
From: almquist@JESSICA.STANFORD.EDU ("Philip Almquist")
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Routing and Multiple Subnets on one net
Message-ID: <9003042322.AA08104@jessica.Stanford.EDU>
Date: 4 Mar 90 23:22:08 GMT
References: <25ed4b52.69c4@polyslo.CalPoly.EDU>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
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Ralph,
> First, we are running multiple (2) subnets on the same physical (DLL)
> network. There are a number of papers that claim this is leagal, but
> perhaps their setup was diferent. In our case the two networks (subnets)
> are 129.65.16.0 and 129.65.160.0, the wrinkle is that the mask on
> 16.0 is ff.ff.f0.00 and on 160.0 it is ff.ff.ff.00 . 

	I don't believe that the IP and subnet specs either explicitly
permit or deny running multiple nets or subnets on the same cable, in large
part (as I understand history) because it didn't occur to their authors that
anyone would want to.  However, it has since become accepted practice in
large parts of the IP community, and I seem to recall that the Host
Requirements RFC's try to ensure that hosts handle this practice correctly.

	Multiple subnet masks on the same net, on the other hand, is still a
very controversial practice, with strong proponents and strong detractors.
One thing that is agreed upon is that using multiple subnet masks requires
great care in choosing masks and assigning addresses in order to avoid the
sorts of problems you report.  I seem to recall that there is some
mathematical analysis of what works and what doesn't in the OSPF spec.
There has also been talk of starting an IETF working group to study the
issue.

> Personaly I would think that any packet that is a broadcast
> at the DLL level should not be automaticaly routed.

	Your view is at odds with the tradional one, but is becoming
fashionable.  My guess is that your view will be incorporated into the next
revision of RFC1009.
							Philip