[4372] in linux-net channel archive
Re: Subnets - Can you have more than one?
daemon@ATHENA.MIT.EDU (Martin Mares)
Sat Sep 14 09:02:11 1996
From: Martin Mares <mj@k332.feld.cvut.cz>
To: danny@ct-ngnet.army.mil (Daniel Maldonado)
Date: Sat, 14 Sep 1996 12:43:38 +0200 (MET DST)
Cc: linux-net@vger.rutgers.edu
In-Reply-To: <01BBA1C3.25EB68A0@DMC_HOME.ngctdpi> from "Daniel Maldonado" at Sep 13, 96 10:30:15 pm
Hi,
> I met a guy at work the other day and started talking about
> subnetting. He said that two computers on the same network can have
> different subnet masks. For example one computer can have a subnet mask of
> 255.255.255.0 while another could have a 255.255.248.0 subnet. I was under
> the impression that there could only be one network number in ever network.
> He said that the computers with different subnets would not be able to see
> each other but they would both be able to share the network and find other
> computers with their same subnet. Is this true? Maybe when it was
> explained in the Cisco router class that I took I misunderstood the
> instructor. Can anyone verify this? I need this info for our Linux
> systems.
It's generally possible to have different netmasks on the same physical
network, but in addition to being useless (well, with exception of few
special situations where it can help), it brings several problems:
(1) The broadcasts generated by the machines with the larger subnet
will be seen by the others, but not vice versa.
(2) The routing from the machines with the smaller subnet to the
others must be defined explicitly, but may lead to ICMP redirects
unless the gateway is told not to send them, which will result in
asymmetric routing.
(3) Things like address mask discovery ICMP's and routing protocols
will get confused.
Martin
