Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!sun-barr!cs.utexas.edu!uunet!mcvax!prlb2!kulcs!stephan
From: stephan@kulcs.uucp (Stephan Biesbroeck)
Newsgroups: comp.protocols.tcp-ip
Subject: Dividing Class B number in subnets, Routing Problem
Keywords: Subnets, Routing
Message-ID: <159@pollux.kulcs.uucp>
Date: 23 May 89 12:47:11 GMT
Organization: Kath.Univ.Leuven, Comp. Sc., Belgium
Lines: 39


We are planning to build a university-wide (=city-wide) network. 
We have an official class B IP-number.
I'd like to divide the IP-number in subnet numbers with a certain
flexibility e.g. giving a great department a subnet number of 21 bits
(= 11 bits left for further subnetting or for their hosts) and giving
a smaller department a subnet number of 25 bits (=7 bits left for further
subnetting or for hosts).
The division should also be hierarchical : all the subnets connected 
to the same router at the backbone have subnet numbers that start with
the same bits.

MY PROBLEM IS THE ROUTING. 
In Comer's book "Internetworking with TCP/IP", there is a paragraph
about routing with subnets (16.10 p.202) : about the routing table :
"each entry contains one additional field that specifies the subnet mask
used with the network in that entry:
     (subnet mask, network addreess, next gateway)

!!BUT, in a lot of implementations (UNIX BSD 4.3, SunOS, ...), there is
no place for a subnetmask for each entry in the routing table. 
The subnet mask is only specified for the interface. But it is possible
that my interface has a subnet mask of 25 bits, and I want to direct
to a subnet with a subnet mask of 21 bits.
How can I solve this ?

I hope my problem is clear (it's hard to explain in a few words).
Anyone who can give some help ?? Also other tips about dividing
a class B address in subnet numbers are welcome.

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Stephan Biesbroeck                        Katholieke Universiteit Leuven
Tel : +32/16/20.06.56(x3602)              Dept. of Computer Science
telex : 23674 KULEUV B                    Celestijnenlaan 200A
fax : +32/16/20.53.08                     B - 3030 Leuven
email : stephan@cs.kuleuven.ac.be         Belgium
        stephan@blekul60.BITNET
        stephan@kulcs.UUCP
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