Path: utzoo!utgpu!watserv1!watmath!att!tut.cis.ohio-state.edu!ucbvax!desktalk.com!rlg
From: rlg@desktalk.com (Richard L. Gralnik)
Newsgroups: comp.protocols.tcp-ip
Subject: An INTERESTING problem
Message-ID: <9101100248.AA01030@desktalk.desktalk.com>
Date: 10 Jan 91 02:48:55 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 66

Hi everyone,  

We are trying to make efficient use of a Class B address in a situation where
the standard procedure wastes roughly half the available addresses.  Any
thoughts on our proposed solution(s) or others of your own devising would be
greatly appreciated.

Here is the scenario:

We have a network which resembles a wheel with spokes - central hub of main
processors on a common ethernet, with remote offices scattered around the
area.  The remote offices are to be linked to the central site over serial
lines using routers.

The standard wisdom/procedure is to assign a subnet number to each remote
office, another subnet number to each serial line, and another (or many)
to the central site net.  We want to use an 8-bit subnet mask for the 
obvious reasons, but the cost of this is that the serial lines become 2-node 
subnets, thereby wasting 251 addresses (including 0 and 255) each.  Since 
there are 20 remote sites, and the user wants redundant serial lines because 
the network is mission-critical, we eat up 60+ subnet numbers right off the
bat.  

This is the pilot for a nation-wide company, so the other subnet numbers are
expected to be needed before too long. Also, this operation is growing and
will likely need additional remote offices added later.

We have thought of 3 solutions -

	1.  Use more than 8 bits for the subnet mask.  We don't like the
		administrative problems this creates, plus there is a 
		good chance that later additions to the net will have
		many hosts per subnet, so the additional mask bits will
		not leave enough host address space.

	2.  Use different size subnet masks for the serial lines than for
		the office subnets.  We don't think the routers will like
		this very much.

	3.  Use our Class B network number for the central net and for the 
		remote office nets with the 8-bit subnet mask, and use
		subnetted Class C addresses for the serial lines.  We think
		this will work since all the Class B subnets have the same 
		net number and subnet mask, and since RIP only sends 
		(sub)net numbers and next hop addresses, the updates should
		be accurate.
		
		We think this is very clever, and believe it will work.  
		However, we haven't actually tried it yet...  Also, we
		aren't knowledgeable enough about OSPF, IGRP, etc. to know
		if it will confuse/trash them.  There is also the 
		consideration that a network address space is assumed to be
		physically contiguous, but in this case, the Class B is 
		fragmented, and the Class C is not directly reachable from
		outside the Class B.  On the other hand, since the only 
		nodes using the Class C addresses are the serial interfaces
		of the routers, maybe none of this matters.

What do you think?

Thanks in advance for any input.   I will summarize for the net if we get
some good responses.


Richard Gralnik
(rlg@desktalk.com)