Great Circle Associates Firewalls
(September 1996)
 

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Subject: C-class net, netmask 255.255.255.128 = trouble?
From: Mikael Suokas <csuokas @ cc . hut . fi>
Date: Thu, 19 Sep 1996 20:24:16 +0300 (EET DST)
To: firewalls @ GreatCircle . COM

Hello,

Note: no need to Cc: me, I will read the firewalls list when
I have waded through the current 300 messages unread...


Firewall relevance:
-------------------

Splitting a C class network in two, in order to create a DMZ.

Question:
---------

What can go wrong if a 255.255.255.128 netmask is used? RFC 950
and Comer's "Internetworking With TCP/IP" recommends not using
subnet masks with all zeros or ones in the subnet's network field.
With one bit of subnetting, it would obviously be "all 1" or "all 0".

Quote from RFC-950:

         It is useful to preserve and extend the interpretation of these
         special addresses in subnetted networks.  This means the values
         of all zeros and all ones in the subnet field should not be
         assigned to actual (physical) subnets.

What would be the best solution, when a subnet of > 60 hosts is needed?
Also, renumbering of the current *.*.*.1 - *.*.*.50 range of machines
is highly undesirable.

regards,

  - Mikael Suokas -

-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-_-~-
                          Mikael .
 Suokas @
 hut .
 fi
                  http://www.hut.fi/~csuokas/index.html



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