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From: long-morrow@CS.YALE.EDU ("H. Morrow Long")
Newsgroups: comp.protocols.tcp-ip.domains
Subject: Re: Subnetting and RFC 1101
Message-ID: <9011131523.AA04083@ALASKA.CF.CS.YALE.EDU>
Date: 13 Nov 90 15:23:05 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: inet
Organization: The Internet
Lines: 37




In article <2858@naucse.cse.nau.edu> root@naucse.cse.nau.edu (Paul Balyoz) writes:

>>Is it legal to use an address for a NETWORK such that the subnet portion
>>is all zeroes?  For example, suppose that the class-B address 134.114
>>has been assigned to you, and you use 8-bit subnetting (netmask
>>is 255.255.255.0).  Does this mean that 134.114.0 can be used as a
>>legal subnetwork, which can have hosts assigned below it (134.114.0.23),
>>or must there be a non-zero value for the subnet portion (3rd octet)?

>While it is technically legal, we avoid it like the plague.  The reasons
>are numerous but basically all come down to, "someday, it will bite you
>in the ass."  Consider that a broken implementation, or one that still
>believes ONLY 0's broadcasts (are there still any of these?) may have
>problems with it.  Since you have 256 subnets to play with, it hurts little
>to loose this one.

This did 'bite us' once.  We have a ZERO subnet ( 128.36.0.0 ) for historical
reasons.  Someday we may change it to 128.36.1.0 if we are forced to, but
we have no masochistic desires of the subnet/host renumbering kind currently.

When we converted from a collection of class C nets to a subnetted class B
we discovered that the HP9000 that was the gateway between our backbone
and our HP 9000 net didn't handle subnetting correctly.  We contact HP and
they told us that HP had written their own TCP/IP kernel code for HP-UX
and it didn't recognized zero subnets.  We had to put our HPs back on a
class C and used RTs with the subnet hack (and Suns with a similar arp hack)
to spoof the HPs and non-4.3bsd subnet capable machines until they could be
converted or discarded.  I don't know if HP-UX still has this aversion to
ZERO subnets.  Allowing ZERO subnets (or not) is an option in the CISCO box
configuration.


--
H. Morrow Long   Long-Morrow@CS.Yale.EDU   {harvard,cmcl2}!yale!Long-Morrow