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: <1990Nov27.025444.15666@bwdls61.bnr.ca>
Summary: Use "service subnet-zero" in our config
Sender: usenet@bwdls61.bnr.ca (Use Net)
Organization: Bell-Northern Research, Ottawa, Canada
References: <1120@soleil.UUCP>
Date: Tue, 27 Nov 90 02:54:44 GMT

In article <1120@soleil.UUCP>, gopstein@sisun1.squibb.com (Rich Gopstein) writes:
> We currently have a flat (non-subnetted) class-B network connected
> via bridges.  At the moment, the number of machines talking IP is
> less than 150, so they are all numbered 140.176.0.x.  We would like
> to start subnetting our network with a subnet mask of 255.255.255.0
> by replacing one of the bridges with a router, changing the netmask,
> and renumbering the machines on the new subnet to 140.176.1.x (leaving
> the machines on the rest of the net numbered 140.176.0.x).
> 
Ooooohhhh.... I guess small *really is* beautiful; read on...

> The problem is that our cisco technical contact (in the local sales office)
> claims that we have to renumber all of the 140.176.0.x machines since
> zero is not legal in the subnetted portion of the address (ie the third
> octet)...  We asked him whether he meant the HOST portion of the address,
> but he said that he really meant the third octet.

That's correct, subnet zero is no longer legal, but it used to be.
I would suggest that your local rep RTFM...  All you have to do is code
 
          service subnet-zero
 
before you attempt to change the subnet mask.  Doing it the other way around
will not be accepted by the cisco.  However, you *should* consider renumbering
those nodes on subnet zero; don't know if cisco will support this feature(?)
(backward compatibility) for much longer.

Consider yourself lucky you only had a Class B network...  We are almost 
finished our Class A conversion (you guessed it!  Subnet *zero* AND about
2000 nodes when we started.  The worst part:  while converting old nodes, 
there were new nodes being added to this old network.  Anyway, 1991 will 
free us of this costly startup mistake.

> 
> This doesn't make sense to me.  Is there a reason that a zero in the
> subnetted portion of the address will cause cisco routers a problem? 
> 
No problem with the zero, but like I said above, seriously consider an orderly
conversion.
> 
> Rich Gopstein
> gopstein@squibb.com
> rutgers!squibb.com!gopstein
> 
Good luck with your cisco router(s),

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