Re: tlug: ip subnetting question..

[Jens-Ulrik Petersen <jens-ulrik.petersen@example.com>]

Jonathan Q <jq@example.com> writes:

> On Fri, 3 Dec 1999, Scott M. Stone wrote:
> > 
> > OK, now I always thought that a netmask of 255.255.255.192 gave you 4
> > subnets of 62 hosts each.  My CCNA book however *insists* that this mask
> > gives you *two* subnets of 62 hosts each.
> 
> I'm glad you asked that question, 'cuz I happen to have a Cisco Press
> book sitting right here  :-)  Inside the front cover is a CIDR
> conversion table, and the relevant line says:
> 
> /26  255.255.255.192  11111111 11111111 11111111 11000000 1/4C
> 
> This supports your belief.  Moreover, on P. 64 of this same book, it
> states specifically that a /26 AKA 255.255.255.192 gives you 4 subnets,
> bearing out the numbers of 11000000.
> 
> The book, if you want to pick it up yourself, is called Internet Routing
> Architectures, by Bassam Halabi.  Published by Cisco Press, ISBN
> 1-56205-652-2.
> 
> You wouldn't think that a CCNA book would be wrong, but . . . 
> 
> Unless they are just referring to only two of the four, it seems as
> though something is fishy there.
> 
> Anybody else got any ideas on this?

>From rfc-950:

---- Start of included text -----------------------8<--- cut here -------------
RFC 950                                                      August 1985
Internet Standard Subnetting Procedure

      Special Addresses:

         From the Assigned Numbers memo [9]:

            "In certain contexts, it is useful to have fixed addresses
            with functional significance rather than as identifiers of
            specific hosts.  When such usage is called for, the address
            zero is to be interpreted as meaning "this", as in "this
            network".  The address of all ones are to be interpreted as
            meaning "all", as in "all hosts".  For example, the address
            128.9.255.255 could be interpreted as meaning all hosts on
            the network 128.9.  Or, the address 0.0.0.37 could be
            interpreted as meaning host 37 on this network."

         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.

            In the example above, the 6-bit wide subnet field may have
            any value except 0 and 63.

---- End of included text -------------------------8<--- and here -------------

Also the IBM Redbook "TCP/IP Tutoral and Overview" 6e by Murhammer et
al (Prentice Hall, 1998)

        <http://www.redbooks.ibm.com/abstracts/gg243376.html>

reiterates this in section 2.1.2.

Jens

[9] currently rfc-1700
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