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From: jkimball@SRC.Honeywell.COM (John Kimball)
Newsgroups: comp.protocols.tcp-ip
Subject: subnetting on non-byte boundaries
Message-ID: <50277@srcsip.UUCP>
Date: 12 Dec 89 22:19:36 GMT
Sender: news@src.honeywell.COM
Reply-To: jkimball@src.honeywell.com ()
Organization: Honeywell Systems & Research Center
Lines: 53


Time to call on the Wisdom of the Net . . .

We have a Class B network.  We've been following the strategy which all the
examples in the manuals depict: subnetting at the byte boundary, using the
third octet for our (internal) network ids and the fourth octet for the
host id.  

We've also been keeping our backbone as one logical network (actually three
ethernet segments joined by learning bridges).  On that backbone network,
we are approaching 250 hosts.  And we expect to need another 75 or so
additional IP addresses within a few months. Ooops.

Looks like our options are:

  Option 1:  Modify our subnetting scheme.  Use 6 bits for internal
     network id, and use 10 bits for host id.  We've kept our network
     id's in the high bits of the third octet, so the low bits are free and
     can be reassigned to the 'host part'.

     Worry 1.1:  The only examples in the manuals show subnetting on byte
       boundaries.  Will a 6/10 (vs 8/8) bit-split really work?  (Context:
       lots of Suns running 4.0.3;  VMS VAXen running CMU-TEK 6.4; some
       random Apollos and HPs that we don't care about, much.)

     Worry 1.2: Can we really expect to keep increasing the number of hosts
       on our backbone network at this rate?  When will performance
       problems set in?

  Option 2:  Play games with routing.  Put two networks on the same
       backbone, both with metric 0.

     Worry 2.1: Is this some sort of unsupported kludge -- will it work,
       for our mix of hosts?

     Worry 2.2: If we should in the future attach some gateways to that
       backbone cable, would they die of terminal confusion?

  Option 3:  Bite the bullet and buy some hardware.  Replace the learning
     bridges with true gateways, making the three segments be three networks.


Has anyone done Option 1 or Option 2?  What would you recommend?

As always, reply via mail and I'll summarize if indicated.

Thanks!
						John Kimball

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