Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!wuarchive!emory!att!bu.edu!purdue!smb
From: smb@cs.purdue.EDU (Scott M. Ballew)
Newsgroups: comp.protocols.tcp-ip.domains
Subject: Re: Subnetting and RFC 1101
Keywords: subnetting rfc 1101 host zero
Message-ID: <12385@medusa.cs.purdue.edu>
Date: 8 Nov 90 14:08:02 GMT
References: <2858@naucse.cse.nau.edu>
Sender: news@cs.purdue.EDU
Reply-To: smb@tristram.cs.purdue.edu (Scott M Ballew)
Organization: Department of Computer Science, Purdue University
Lines: 99

In article <2858@naucse.cse.nau.edu> root@naucse.cse.nau.edu (Paul Balyoz) writes:

>Is it legal to use an address for a NETWORK such that the subnet portion
>is all zeroes?  For example, suppose that the class-B address 134.114
>has been assigned to you, and you use 8-bit subnetting (netmask
>is 255.255.255.0).  Does this mean that 134.114.0 can be used as a
>legal subnetwork, which can have hosts assigned below it (134.114.0.23),
>or must there be a non-zero value for the subnet portion (3rd octet)?

While it is technically legal, we avoid it like the plague.  The reasons
are numerous but basically all come down to, "someday, it will bite you
in the ass."  Consider that a broken implementation, or one that still
believes ONLY 0's broadcasts (are there still any of these?) may have
problems with it.  Since you have 256 subnets to play with, it hurts little
to loose this one.


>How about 255 for the 3rd octet?  Seems like this should be OK,
>because your broadcast address would be 134.114.255.255, which,
>with the aforementioned netmask, is only broadcast for the network
>134.114.255 .

However, this is also the "all subnets" broadcast for net 134.114.
What do you do now?  Again, we avoid this one and have now lost only 2
of 256 possible subnets.  This has not been a problem.

>If this is legal, then I see a problem with the extensions described
>by RFC 1101.  Suppose addresses 134.114 are subnetted 255.255.255.0,
>but then subnet 134.114.0 is subnetted FURTHER: netmask 255.255.255.248!

This becomes dangerous (different subnet masks for the same network).
Consider the following topology:


    A        B                        
    |    1   |
    +--------+
             |
             C
          2  |
        +----+
        |     
        D     	    	    E----The rest of the Internet
        |          3        |
        +----------+--------+
                   |
                   F
               4   |
            +------+
            |
            G

Now, assign the following IP network/host addresses and masks:
Net 1:  128.10.4.0      255.255.255.0
Net 2:  128.10.3.128    255.255.255.128	    (7 bit host part)
Net 3:  128.10.3.0      255.255.255.128
Net 4:  128.10.2.0      255.255.255.0

Host A: 128.10.4.1
Host B: 128.10.4.129
Host C: 128.10.4.2, 128.10.3.130
Host D: 128.10.3.131, 128.10.3.3
Host E: 128.10.3.4, (some address that doesn't matter)
Host F: 128.10.3.5, 128.10.2.5
Host G: 128.10.2.6

Now consider the case when host G wishes to send a datagram to host B.
It consults its routing table and routes it to F.  F consults its
routing table and finds an entry for net 128.10.4.0.  What subnet mask
does it use?  The one for subnet 2?  Subnet 3?  If it chooses the mask
for subnet 2, it will correctly route this datagram to gateway D.  If
it chooses the mask for subnet 3, it will incorrectly route this
datagram to gateway E (default route).

Assume it chose correctly and sent it to D.  D consults its routing
tables and also finds an entry for 128.10.4.0.  However, it has only
one subnet mask to choose from, namely the one for subnet 3.  It
therefore incorrectly routes the message to gateway E and sends a
redirect to gateway F.

Assume, now, that gateway F sent the datagram to gateway E or that
gateway D forwarded it to gateway E.  E consults its routing table and
discovers an entry for 128.10.4.0.  If it chooses the 9-bit mask of
its one and only subnet, it will incorrectly send a destination
unreachable message to host G.  If, on the other hand, it somehow
comes up with the correct 8-bit mask, it will route the datagram to
gateway D (who will surely send it back to E).

Now the question:  Is it really worth it to run different subnet masks
on the same network?

>Paul Balyoz, Software Specialist II    pab@naucse.cse.nau.edu
>Northern Arizona University            root@naucse.cse.nau.edu
>PO Box 15600, Flagstaff, AZ 86011      pab%naucse@arizona.edu
>NICname for whois: PB1                 ...arizona!naucse!pab

Scott Ballew
Cypress Network Operations Center
Purdue University Department of Computer Sciences