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