[Gllug] subnetting

[Ian Northeast]

Paul Cupis wrote:
> On Thursday 11 November 2004 19:16, "Paul Cupis" <paul at cupis.co.uk> 
> wrote:
> 
>>I'll look around for some reference to this later.
> 
> 
> Aha, found it:
> 
> http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml#subnetzero
> 
> Traditionally, it was strongly recommended that subnet zero and the 
> all-ones subnet not be used for addressing. According to RFC 950, "It 
> is useful to preserve and extend the interpretation of these special 
> (network and broadcast) 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." This is the reason why network 
> engineers required to calculate the number of subnets obtained by 
> borrowing three bits would calculate 23-2 (6) and not 23 (8). The -2 
> takes into account that subnet zero and the all-ones subnet are not 
> used traditionally.
> 
> RFC 950
> http://www.ietf.org/rfc/rfc0950.txt

Thanks for that, it's making sense now. That RFC is 19 years old! And 
the practice has been obsolete as per RFC1878 for at least 9 years.

I hope that if universities are teaching this ancient stuff they are 
making clear that it's a history lesson:)

But according to RFC 950, the top subnet shouldn't be used either, so 
there would only be 6 left (as Mark Green said, but didn't follow up 
with the reasoning). It appears that Cisco were a bit inconsistent when 
implementing that feature in IOS (but then Cisco are often inconsistent 
so no surprises there).

Now that you mention it, Michael did mention an out of date Cisco 
router. I didn't take a lot of notice as it wasn't one I had heard of. I 
suppose I naively assumed that a Cisco router would work properly. I 
really don't know why come to think of it, given some of the experiences 
I've had with Cisco kit. So Michael, if this is one of these routers 
with this "feature", the answer to your original question is contained 
in the link to the Cisco site above.

Has all this been explained in your class?

Well, you learn something every day as they say. How useful this 
particular something is I'm not quite sure, but it's certainly 
interesting. Thanks for doing that research, Paul.

Actually I learned something else today, which is useful, but it's about 
AIX so I'll get my coat now.

Regards, Ian

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