Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!VENERA.ISI.EDU!braden
From: braden@VENERA.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Subnetting
Message-ID: <8805091611.AA07124@braden.isi.edu>
Date: 9 May 88 16:11:40 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 45


	
	> >  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.
	
Stuart,

Let's consider the three examples you cite.  

Network Redirect:  It is recognized that network redirects are a problem
in a subnetted environment, and therefore the Gateway Specification
RFC-1009 says that gateways should only be sending host redirects.
If you have a gateway within your subnetted environment that is sending
network redirects, it should be brought up to spec.

Synthesizing a remote broadcast -- presumably you mean a directed
broadcast. Yes, this is a real engineering problem, although it is an
application-level problem, not an IP level problem.  If you have an
application that is sending directed broadcasts into another subnet of
the same network, that application needs some configuration information
-- obviously, it needs the remote subnet number.  You might as well
configure it with the complete 32-bit Internet directed broadcast
address.  Synthesizing IP addresses should be avoided whenever possible.

Routing to a remote subnet -- This is entirely a gateway problem.
The solution, as RFC-1009 suggests, is simply to include the
subnet masks with the network numbers in the routing updates among
the subnet gateways.  I wonder why no one has extended RIP in this
obvious way yet.  As you say, it is merely a matter of a little
engineering.   
   
After all this, I would challenge your statement that "in general,
you -do- have to be able to know the sizes of sibling subnets"
(at least, if "you" is a host, not a gateway).

Bob Braden