Classless Reverse zones

[Bob Vance]

Thanks for the detailed explanation.

It's amazing.  I've been networking for years and understand prefixes
and CIDR, but I've just never heard (at least I don't remember :) the
term "bit alignment" before, even though understanding and using the
concept !
Of course I don't deal with CIDR daily -- maybe it's a common term in
those circles dealing with address aggregation and route summarization.


>If the numbers are not bit aligned, they don't share a common
>prefix and thus can't be supernetted by shifting the netmask
>to the left.

Right, but I think that that really had nothing to do with the DNS issue
at hand except that this was the set of numbers that had to be delegated
individually for a given prefix.  In fact, in this case, we weren't
"supernetting" at all (network 10), if we want to use that term and make
a distinction between it and subnetting.
Hmmm.  Well, OK.  I guess since the issue arose out of having a split
3rd octet and things work better at the natural /24 boundary, then, yes,
we *were* supernetting with respect to /24 :)



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BV         | <mailto:BobVance at alumni.caltech.edu>
Sr. Technical Consultant,  SBM, A Gates/Arrow Co.
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-----Original Message-----
From: Andreas S. Oesterhelt [mailto:oes at oesterhelt.dyndns.org]
Sent: Friday, March 16, 2001 4:48 AM
To: bobvance at alumni.caltech.edu
Subject: RE: Classless Reverse zones


Bob Vance writes:
>
> Thanks for the reply, but it did nothing to advance my understanding
of
> what you mean by bit aligned :|

Numbers are bit-aligned when they share a common binary prefix, so
that no more than the log n least significant bits differ, for n
consecutive numbers.

0,1,2,3 are aligned because their representations all start with 6
zeroes (in 8 bit) and have log 4 = 2 variable bits at the end.
1,2,3,4 differ in the last 3 bits, so they're not.

> And I don't know how it relates to the original question.

If the numbers are not bit aligned, they don't share a common
prefix and thus can't be supernetted by shifting the netmask
to the left. Since you said you had a 22bit mask, this might
have been 4 neighboring Class C networks.

Regards,
--Andreas