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From: cudcv@warwick.ac.uk (Rob McMahon)
Newsgroups: comp.protocols.tcp-ip,comp.dcom.sys.cisco
Subject: Partial subnetting
Message-ID: <=~P_0Q&@warwick.ac.uk>
Date: 14 May 91 16:05:03 GMT
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Organization: Computing Services, Warwick University, UK
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We have a class B address (137.205.0.0) for the University.  Currently the
whole campus is running without subnets, with bridges between each department
and the backbone, but we allocated the numbers such that we could later give
each department its own subnet, and change the bridges for routers should it
become necessary.

We now have our first department that actually wants to be split off with a
router, and I realise that I don't understand how it works at all.  Here was
the plan (internet address/netmask):

			    137.205.0.0/0xffff0000
				      |
				      |
			 ----------------------------
			 |                          |
		      Bridge			 Router
			 |  			    |
              137.205.232.0/0xffff0000	 137.205.176.0/0xfffffc00

But the Cisco router we had on trial won't let you set different netmasks on
its two interfaces "in the current implementation".  Does anyone know if this
is likely to change ?

A nearby book calls what we are trying to do an "illegal" setup, and says it
is "recommended" to have the same netmask throughout the network.

It seems that to make this work the routing tables on the Unix hosts ought to
have a netmask associated with each entry, but it's not there.  When I tried
to add a route to such a gateway the machine lost contact with the entire
world.  (I still don't understand this, if the machine was just picking the
wrong route and sending all packets to the router, why didn't the router just
forward them to the correct machine ?)

Is this setup not supposed to work ?  Why not ?  It seems like an obvious
application (so obvious that I just assumed it was going to work without
really sitting down and thinking about it).  We don't want to have to subnet
the entire University just yet, but it would be nice to keep our options open.

Cheers,

Rob
-- 
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Rob McMahon, Computing Services, Warwick University, Coventry CV4 7AL, England