Path: utzoo!mnetor!uunet!husc6!bloom-beacon!gatech!purdue!decwrl!ucbvax!UC.MSC.UMN.EDU!slevy
From: slevy@UC.MSC.UMN.EDU ("Stuart Levy")
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Subnetting
Message-ID: <8805090727.AA11410@uc.msc.umn.edu>
Date: 9 May 88 07:27:22 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 25

> >  b) "Subs" of a given "whole" must be of equal size.
>  
> This is a mistaken assumption.  There is nothing that prevents you
> from using subnets of different sizes on a given net, except for
> software that isn't up to speed on subnetting (notably SunOS 3.x).

Unfortunately, there -are- problems with dividing a network into
variable-sized subnets -- not just incomplete software implementations
but real engineering problems.  They relate to cases where hosts or
gateways need to know the size of a subnet they're not attached to:
e.g. when interpreting an ICMP network redirect, synthesizing a remote
broadcast address, or routing to a remote subnet.

You may be able to live with the effects of misinterpretations if things
are kept simple enough (so long as nothing sends you a network redirect :->).

The subnetting RFCs, e.g. 917, 936, 950 discuss some possible conventions
for determining subnet sizes, including equal sizes on a given network (easy)
and self-encoding subnet sizes analogous to the class A/B/C sizing for ordinary
networks.  I suppose it would also be possible to distribute a table of
subnet sizes to every host and gateway on a network.  But in general,
you -do- have to be able to know the sizes of sibling subnets, and the
equal-size case seems to be the closest one to a standard.

	Stuart Levy