Re: Binary Math (Part II) & IPv4 Prefix Summarization

Looking at this example:168.192.3.0/24
168.208.11.0/24
168.208.3.0/24
168.192.11.0/24

The first octet is the same. So we ignore that one.

Second octet has 192 and 208. (208-192 = 16) The third octet has 3 and
11. (11-3 = 8).

With a mask of 0.16.8.0, you have a total of TWO bits in the 1-position.
2^2 = 4 possible matches. You have four matches in your list. So all
four values ARE matched.

access-list 21 permit 168.192.3.0 0.16.8.0

That's your answer by itself. Now, unfortunately, I haven't read through
Petr's method... He's a genius, so I believe it would be quite good.
However, the answers you came up with using that method are incorrect.
So I'm not sure what to tell you about that part, or how you applied the
method that something may have been missed! Math really doesn't lie. So
as long as we apply the correct numbers and match the correct binary
values, we should always come up with the same answer!

Cheers,

Scott

Tannie Philly wrote:

  Dear Scot\Petr, I want to sincerely appreciate the the two blogs
  below where you both dissected the subject of Summarization using
  different methods. I learnt alot. They were great!
  http://blog.ine.com/2008/11/03/binary-math-part-ii/
  http://blog.ine.com/2010/03/17/a-simple-ipv4-prefix-summarization-procedure/
  I decided to compare both methods to see if I will arrive at same
  result. But please I need a little more light on the difference I
  noticed. Looking at this example:168.192.3.0/24
  168.208.11.0/24
  168.208.3.0/24
  168.192.11.0/24 Using the Binary Method taught by Scot the result was
  access-list 21 permit 168.192.3.0 0.16.8.0
  access-list 21 permit 168.192.11.0 0.16.0.0 But using Prefix
  Summarization taught by Petr the result was
  access-list 21 permit 168.192.3.0 0.32.16.0access-list 21 permit
  168.192.11.0 0.32.0.0
  I am alittle-bit confused.Please kindly clarify, are these results
  the same? Which method should someone stick to? Do they have
  advantages over the other? Tannie

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Received on Wed Mar 24 2010 - 09:38:23 ART