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From: kre@munnari.oz.au (Robert Elz)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: Serial interfaces IP number question
Message-ID: <25011@boulder.Colorado.EDU>
Date: 22 Aug 90 00:22:21 GMT
Sender: daemon@boulder.Colorado.EDU
Lines: 38


    Date:        Mon, 20 Aug 90  21:13:27 EDT
    From:        "Roger Fajman" <RAF@CU.NIH.GOV>
    Message-ID:  <9008210111.AA08964@alw.nih.gov>

    We did that, but it didn't work out very well in practice.  The problem
    is that the subnet mask exists in so many places (routers and hosts)
    that people get very upset when you want to change it.

Obviously with a scheme like this you don't want to have to
change all the netmasks every time that you add a few more
subnets - the sensible approach is to numbre subnets from the
high order bits, hosts form the low order bits, and set the
netmask at some appropriate place in between (based on expected
usage of the number space).

The mask would only ever need to be altered if the anticipated
usage of the number space turned out to be incorrect.

    So we finally settled on a 7 bit subnet mask and got a
    second class B address to use when we run out of subnets.

If you have > 254 hosts on any nets, then it looks (currently)
as if you made a wise choice, which probably means that you
made it with sufficient useful information.  Sites starting
out with their first 5 hosts or so, on 3 or 4 subnets are
rarely able to predict that accurately.

If your subnet space holds out until your routers implement
variable width masks, then most of your subnets will need to
have more than 254 hosts before you would need a second net
number.

Personally, I'm hoping that variable width subnet support
will become mandatory in the forthcoming router requirements
RFC.

kre