Newsgroups: comp.dcom.sys.cisco
Path: utzoo!utgpu!cunews!bnrgate!bwdls61.bnr.ca!bwdls56!fortinp
From: fortinp@bwdls56.bnr.ca (Pierre Fortin)
Subject: Re: Subnet 0
Message-ID: <1990Dec1.071411.17026@bwdls61.bnr.ca>
Sender: usenet@bwdls61.bnr.ca (Use Net)
Organization: Bell-Northern Research, Ottawa, Canada
References: <1120@soleil.UUCP> <1990Nov27.025444.15666@bwdls61.bnr.ca> <20599@crg5.UUCP>
Date: Sat, 1 Dec 90 07:14:11 GMT

In article <20599@crg5.UUCP>, edb@crg8.sequent.com (Edward Bunch) writes:
> 
> I've heard this subnet 0 stuff before but I don't understand *exactly*
> what they mean by it. I can see the example of a class B being
> subnetted 8 bits pretty clearly.
> 
> ( 138.95.X.Y ; X nor Y can be 0 )
> 
> Y I really understand, no hosts can be 0.  X isn't so clear. Doesn't
> all this 138.95 stuff on the left keep the network from being 0?

The issue is *SUB*net 0, therefore, in your example, that's X and X alone.
> 
> What if we are doing something weird like subnetting 9 bits?
> Which address configurations do I avoid now?

It's all quite simple if you convert everything to bits.  With a 9/7 split,
  138.95.0.0 is still obvious, but
  138.95.0.128 may not be.  This one is illegal because the Y (7 bits) is 0.
               Its X is actually 00000000.1xxxxxxx; while
  138.95.0.129 is the first legal address:  00000000.1xxxxxxx for subnet
                                        and xxxxxxxx.x0000001 for host.
If you follow this through, you will find that the legal ranges are:
  138.95.0.129 - 138.95.0.254 (138.95.0.255 is subnet 1's broadcast)
  138.95.1.1   - 138.95.1.126 
  138.95.1.129 - 138.95.1.254
  138.95.2.1   - 138.95.2.126
  138.95.2.129 - 138.95.2.254
  .
  .
  .
  138.95.254.1 - 138.95.254.126
  138.95.254.127 - 138.95.254.254
  138.95.255.1 - 138.95.255.126  THE END...
Addresses beyond would result in the subnet being all 1's.  My rule is very
simple:  always convert to binary and look for all 1's or all 0's in either
field.  The RFCs actually allow for non-contiguous bits to identify the 
subnet.  Consider the case where the subnet mask for 138.95.x.x is
  11111111.11111111.11xx11x1.xxx1xx11; if you can work this one out, you
will understand fully.  Well, actually consider why either the subnet portion
AND the host portion of the address can NEVER be less that 2 bits each if
subnetting is in use.  Now you understand it all...  :^)

> 
> 		Ed
Happy bits,

Pierre Fortin       Bell-Northern Research     I know, my postings are
Internet Systems    P.O.Box 3511, Stn C        terse and humourless. So?
(613)763-2598       Ottawa, Ontario            RIP: aptly named protocol
fortinp@bnr.ca      Canada    K1Y 4H7          AppleTalk: Adam&Eve's design