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From: tmallory@BBN.COM
Newsgroups: comp.protocols.tcp-ip
Subject: Re: An INTERESTING problem
Message-ID: <9101150036.AA10190@ucbvax.Berkeley.EDU>
Date: 14 Jan 91 16:06:10 GMT
References: <1991Jan10.194655.19685@Think.COM>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 31


In article <9101100248.AA01030@desktalk.desktalk.com> rlg@desktalk.com
(Richard L. Gralnik) writes:
>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.  

Another obvious approach is to use two sizes of subnet masks. Given the
minimum usable size of two bits(00 unused, 11=broadcast, 01 and 10 the hosts),
you can get 64 trunks worth of subnets out of each subnet with an 8-bit host
space.  This is fairly efficient, and allows for use of the conventional
addressing procedure.  Of course, once you have the hierarchical subnetting
then you have the option of giving less than 254 host addresses to smaller
sites.  BBN T/20s give you this option, others probably do as well.

Paul Tsuchiya wrote a paper on assigning addresses in a hierarchical manner
that allowed for expansion of chunks of the address space without forcing host
addresses to change.  The basic idea was to assign addresses right-to-left,
starting with large masks that could be shrunk to expose more of the host
address space.  It's single most important requirement was support for
variable length subnet masks.  I'm pretty sure that it did NOT require
non-contiguous subnet masks, just a general left-to-right hierarchy.  I can't
locate the paper now, but I'm sure someone(Paul?), can supply a pointer to it.

Tracy Mallory
BBN Communications