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From: lars@spectrum.CMC.COM (Lars Poulsen)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Can subnets be separated by another net?
Message-ID: <1990Jul10.003628.5859@spectrum.CMC.COM>
Date: 10 Jul 90 00:36:28 GMT
References: <9007090939.AA11396@decpa.pa.dec.com> <9007091856.AA01804@cincsac.arc.nasa.gov>
Organization: Rockwell CMC
Lines: 26

In all our pride about how OSPF will allow you to build a network that
contains discontiguous subnets, lets not forget to explain to the new
people that this is not recommended, precisely because the earlier and
most widely available routing protocols do not deal well with such a
case.

Disjoint subnets can only be expected to work if all interconnections
between the disjoint subnets are under the same administration and is
running a routing protocol that passes the mask. The normal way to
implement the topology that was asked about, would be to have the
connecting network (C1 in the example) be anotehr class-C sized subnet
of the same class B network as the disjoint segments.

The most common case where clients ask about disjoint subnets, is where
an enterprise is geographically disjoint (say offices in Los Angeles,
Denver and Boston) and wants to attach each office separately to the
Internet, while assigning all host addresses out of the same class B
network number. This of course is utterly undesirable (would OSPF allow
it to be set up at all ?) and contravenes all the intentions for which
subnets were invented.

Thus, for "commercial use" the simple, practical and almost true answer
is that disjoint subnets are not allowed.
-- 
/ Lars Poulsen, SMTS Software Engineer
  CMC Rockwell  lars@CMC.COM