Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!hellgate.utah.edu!basset.utah.edu!haas
From: haas%basset.utah.edu@cs.utah.edu (Walt Haas)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: Sub-Netting Help
Message-ID: <1991Mar17.204830.5973@hellgate.utah.edu>
Date: 18 Mar 91 03:48:30 GMT
References: <33369@boulder.Colorado.EDU>
Organization: University of Utah CS Dept
Lines: 20

In article <33369@boulder.Colorado.EDU> Allen Robel <robel2@mythos.ucs.indiana.edu> writes:
>>someway to tell the router that there are multiple Class C network on 
>>each interface... what happens to a packet 
>>from 128.123.1.x to 128.123.3.x?
>
>IP devices on different subnets on the same interface will
>use the cisco to talk to one another even though they
>are on the same physical cable.

Note that the two networks specified in the example are *NOT* actually
Class C networks, they are subnets of the same Class B network.  In this
particular case (which we support a lot), it is possible to give all hosts
on the physical cable a subnet mask of 255.255.0.0 .  If you do this these
hosts will ARP for any destination on the Class B network, and the cisco
will give a proxy ARP response for any host not on the same cable.  Therefore
the cisco will not have to duplicate packets between hosts that can talk
directly.  The downside, of course, is that there will be more broadcast
packets on that cable.

-- Walt Haas    haas@ski.utah.edu