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

Scott,

You are a gifted Instructor. Well-done!

Your explanation was great and I have also been able to figure out that
Petr's method too was also correct. It's own was more like a summary that
includes all the prefixes while yours matches all prefixes with the minimum
number of entries.

Sounds alike but different.

Once again, thank you.

Tannie
On Wed, Mar 24, 2010 at 2:38 PM, Scott Morris <smorris_at_ine.com> wrote:

> 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 0.32.16.0
> access-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 - 15:10:02 ART