Path: utzoo!attcan!uunet!lll-winken!ames!sun-barr!rutgers!gatech!galbp!wittsend.LBP.HARRIS.COM!mhw
From: mhw@wittsend.LBP.HARRIS.COM (Michael H. Warfield (Mike))
Newsgroups: comp.protocols.tcp-ip
Subject: Re: subnet mask problem
Message-ID: <7962@galbp.LBP.HARRIS.COM>
Date: 25 May 89 14:08:19 GMT
References: <6200023@hpindda.HP.COM>
Sender: news@galbp.LBP.HARRIS.COM
Reply-To: mhw@wittsend.UUCP (Michael H. Warfield (Mike))
Organization: Harris/Lanier Network Knitting Circle
Lines: 48

In article <6200023@hpindda.HP.COM> tozz@hpindda.HP.COM (Bob Tausworthe) writes:

>However, because the algorithm for using masks is to perform a logincal AND 
>and test for equality, technically 255.255.0.255 could be used as a mask.
>Or could it?

>1) is a mask such as 255.255.0.255 even legal (i.e. conform to specifications)

	From RFC 950 (p6):

:      For example, on a Class B network with a 6-bit wide subnet field,
:      an address would be broken down like this:
:
:                           1                   2                   3
:       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
:      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
:      |1 0|        NETWORK            |  SUBNET   |    Host Number    |
:      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
:
:      Since the bits that identify the subnet are specified by a
:      bitmask, they need not be adjacent in the address.  However, we
:      recommend that the subnet bits be contiguous and located as the
:      most significant bits of the local address.

	So yes it is legal, but no it is not a good idea per the
recommendation.  You may even cause serious network problems due to bugs
in some vendors implimentations.  A while back, VAX's managed to send a
mangled subnet mask in reply to an Address Mask ICMP which resulted in
interesting effects in SUN systems.  It even resulted in a new term in
our vocabulary - Broadcast Storm :-).

	Some vendors of tcp/ip code for PC's (Hi FTP) specify the subnet
in their configuration as "n" bits, where n is the number of bits in the
subnet field.  This software obviously is only going to work where the
subnet mask is contiguous and adjacent to the network address.  The
alternative to all of this is proxy arp, where these hosts don't even
know they are subneting.  The subneting is handled and hidden by your routers.
I don't know of ANY routers that will handle obscure subnet like what you're
asking about though, but that certainly doesn't mean there aren't any!  I won't
get into any arguments over who is broke or who is supporting what
recommendations, after all we still got vendors out there that don't even
support subnetting outside of proxy arp!

---
Michael H. Warfield  (The Mad Wizard)	| gatech.edu!galbp!wittsend!mhw
  (404)  270-2123 / 270-2098		| mhw@wittsend.LBP.HARRIS.COM
An optimist believes we live in the best of all possible worlds.
A pessimist is sure of it!