Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!yale!husc6!encore!mperlman
From: mperlman@Encore.COM (Mark Perlman)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Multiple subnets on same cable
Keywords: subnet IP ICMP fnord
Message-ID: <13241@encore.Encore.COM>
Date: 13 Nov 90 14:46:21 GMT
References: <freiss.658073618@peun33> <martino.658404029@krypton>
Reply-To: mperlman@encore.Com (Mark Perlman)
Organization: Independent Consultant
Lines: 40

In article <martino.658404029@krypton> martino@logitek.co.uk (Martin O'Nions) writes:
>freiss@nixdorf.de (the hacker) writes:
>
>
>>This might be a silly question, but it's been worrying me for some time.
>>What happens if different IP subnets live on the same ethernet cable?
>
>>I'm worried about excessive ICMP traffic (ICMP net_redirect packets);
>>I see some SGI workstations here react to some packets from a
>>different subnet with this ICMP message.
>
>>Could somebody tell me if this is a problem or what other
>>problems I haven't even thought of might arise?
>>Pointers to books, RFCs etc. on this subject are also welcome.

I may have missed some of the previous posting in response, however,
I'll put in my 2 cents for free.

You can "turn off" subnetting by specifying a metric of 0 (zero) when
you use the route command.  E.G.:
    route add <some_subnet_address> <some_host_address_in_your_subnet> 0

You also need to consider broadcasting addresses on this
multi-subnetted physical network.  A broadcast address of xx.xx.xx.255
says "all hosts on subnet xx.xx.xx.0", whereas, a broadcast address of
255.255.255.255 says "all hosts an my local network".  It requires some
forethought on your part to assure a working broadcasts.

I would reccommend getting your hands on Charles Hedrick's paper,
"Introduction to Administration of an Internet-based Local Network", 3
October 1988.

Hope this was useful.

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