Newsgroups: comp.dcom.sys.cisco
Path: utzoo!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!think.com!barmar
From: barmar@think.com (Barry Margolin)
Subject: Re: Sub-Netting Help
Message-ID: <1991Mar18.171211.5465@Think.COM>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
References: <33369@boulder.Colorado.EDU>
Date: Mon, 18 Mar 91 17:12:11 GMT

In article <33369@boulder.Colorado.EDU> robel2@mythos.ucs.indiana.edu (Allen Robel) writes:
>cisco supports multiple subnets on each interface with the 
>interface config command
>IP ADDRESS <address><subnet-mask> SECONDARY
>
>We've had up to 4 subnets off a single interface with no 
>problems.  There's no mention in the docs as to what
>the maximum number that cisco supports.

I'm using this, and I've run into a couple of minor problems.

One problem is that there is still only one broadcast address per
interface, so you have to use 255.255.255.255 as the broadcast address,
rather than the subnet broadcast addresses.  This means that hosts on both
logical subnets will receive broadcasts intended only for those on just one
of the subnets.

In particular, this happens with RIP route broadcasts.  Our Unix hosts'
consoles and logs are full of messages "packet from unknown router,
131.239.32.250", which is the address of the router on the other logical
subnet.
--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
{uunet,harvard}!think!barmar