Re: Subnetting/Routing

From: Moe Trin (ibuprofin_at_painkiller.example.tld)
Date: 05/13/05 

Date: Thu, 12 May 2005 19:01:27 -0500

In article <slrnd86hmd.g05.u.volmer@xerxes.is-a-geek.net>, Ulf Volmer wrote:
>Shadow_7 <wwwshadow7@yaNOhoo.comNULL> schrieb:
>>>>What A dim wad so use 255.255.253.0
>>>
>>> What a dim wad ... 255.255.253.0 is not a valid netmask.
>>
>> AFAIK, netmask is a bitwise operator. So it would be valid.
>
>It's not valid.

Actually, it is valid as far as RFCs are concerned. For your reading
enjoyment, see RFC0917, RFC0932, RFC0936, and RFC0940, which were the
conceptual documents, and then read RFC0950 which is the current
authoritative standard.

0950 Internet Standard Subnetting Procedure. J.C. Mogul, J. Postel.
     Aug-01-1985. (Format: TXT=37985 bytes) (Updates RFC0792) (Also
     STD0005) (Status: STANDARD)

specifically relating to the diagram on page 5 of that document. From the
glossary in the back (because it's easier to read)

   Subnet Field
 
      The bit field in an Internet address denoting the subnet number.
      The bits making up this field are not necessarily contiguous in
      the address.
 
and also RFC1122

1122 Requirements for Internet Hosts - Communication Layers. R.
     Braden, Ed.. October 1989. (Format: TXT=295992 bytes) (Updated by
     RFC1349) (Also STD0003) (Status: STANDARD)

specifically, the last sentence of the third paragraph of section 3.2.1.3.

            This notation is not intended to imply that the 1-bits in an
            address mask need be contiguous.

Notice that RFC1122 is also defined as a standard, and is current. Now, the
"real world" is described in RFC1219

1219 On the assignment of subnet numbers. P.F. Tsuchiya. Apr-01-1991.
     (Format: TXT=30609 bytes) (Status: INFORMATIONAL)

which does indeed _recommend_ a contiguous mask (which is what any
sane O/S now expects), BUT notice that this document is rated as
"INFORMATIONAL", not a "STANDARD" as are the others.

Bottom line - you can use a non-contiguous mask on your network if you
please (though it won't be easy to configure), because your internal
mask values are not seen on the Internet. Your network, your configuration,
your problems.

        Old guy

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