Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!sun-barr!decwrl!mogul
From: mogul@decwrl.dec.com (Jeffrey Mogul)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Subnet 0
Message-ID: <188@jove.dec.com>
Date: 20 Sep 89 23:14:50 GMT
References: <41@cvbnet.Prime.COM> <8909151911.AA14633@uf.msc.umn.edu>
Organization: DEC Western Research
Lines: 25

In article <8909151911.AA14633@uf.msc.umn.edu> fin@UF.MSC.UMN.EDU ("Craig Finseth") writes:
>
>	Is it [subnet number = 0] "allowed" (i.e., technically legal)?
>
>Undefined.  Network number 0 means "this net."  By analogy subnet
>number 0 means "this subnet."  You won't find this mentioned anywhere
>official, that's why it is undefined.

Actually, it is mentioned "somewhere official".  From RFC950:

						         [The] values
         of all zeros and all ones in the subnet field should not be
         assigned to actual (physical) subnets.

The Host Requirements (draft?) RFC more or less reiterates this rule.

As an aside: much as it pains me to admit it, in all honesty I have
to say that one probably would not run into any real problems by
assigning a subnet number = 0.  Avoiding subnet 0 is "good engineering
practice" more than a result of bitter experience.  (That is not the
case with assigning a host number = 0, since this looks to some hosts
like a broadcast).  I make this admission because otherwise people
are going to say "I tried using subnet 0, it works, so why the rule?"

-Jeff