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From: equek@hydro.on.ca (Erone Quek)
Newsgroups: comp.dcom.sys.cisco
Subject: Re:  backplane backbone, routing or bridging?
Message-ID: <9106230159.AA01973@ohmg1.hydro.on.ca>
Date: 23 Jun 91 01:59:31 GMT
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In-Reply-To: <9106210211.AA09464@mickey.ycc.yale.edu>; from "Watt-Alan@mickey.ycc.yale.edu" at Jun 20, 91 10:11 pm
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>>> Some previous part has been left out.

> 
> However you do not have to assign a whole subnet to each point-to-point
> ethernet.  You can have a single subnet which is shared by all the
> point-to-point ethernets on both ends.  Assuming you subnet a class B
> on an 8-bit boundary, and the "point-to-point cloud" is subnet 100,
> you could define the subnet mask 255.255.255.252 to allow 6 bits
> of "sub-subnet" and 2 bits of "host" (0 is reserved, 3 is broadcast,
> 1 and 2 are the two endpoints.
>

>>> The example/table has been left out.

> 
> There you have your 20 ethernet point to point links between the AGS+ and
> the 20 other routers, and plenty of address space left in subnet 100 for
> more.
> 
> 	- Alan S. Watt
> 	  High Speed Networking, Yale University
> 	  Computing and Information Systems
> 	  Box 2112 Yale Station
> 	  New Haven, CT  06520-2112
> 	  (203) 432-6600 X394
> 	  Watt-Alan@Yale.Edu
> 
> 
> Disclaimer:  "Make Love, Not War -- Be Prepared For Both"
> 		- Edelman's Sporting Goods [and Marital Aids?]
> 

Alan (& cisco),

I thought one can not has multiple subnet masks across the same
Class B network, i.e. one cannot have 14 bit subnet mask in certain 'part'
of the Class B (in your example: subnet cloud 100) and use 8 bit mask
in other 'part' of the Class B network. I was assured by cisco's customer
engineers that multiple subnet masks within the same class B is something I
DON'T want to get into. As a matter of fact, we were in a very similar
situation. We ended up 'wasting' a full (8-bit) subnet between a pair of
'interconnecting' routers. 

cisco people claim they are working on supporting 'variable subnet masks'.
But I am not as optimistic since that would mean a fundamental change
in their way of distributing IGRP tables (and may be routing decisions
too ?). 

One more problem, how are you going to tell your unix boxes (Sun for example)
about your variable subnet masks (assuming one can NOT use default routing
due to security reasons) ?

Erone Quek
ISD Network Computing Operations
Ontario Hydro
Tel: +1 416 592 6460
EMail: Erone.Quek@hydro.on.ca        or     ...!utcsri!ohmg1!equek