Bitwise Anding question

jlhct · June 2008

Hello,

I'm starting my preparation for 70-291 and I've come across something in my Syngress book that I don't understand:

If i'm bitwise anding these 2 #'s: 146.64.160.9 and 255.255.224.0 why is the result = 146.64.160.0

Am I not understanding something along the way?

Thanks!

dynamik · June 2008

What did you expect it to be?

Write them both out in binary if you have trouble visualizing it. Both bits you're ANDing have to be a 1 in order to have the result be 1. Since the last octect is all 0s, all ANDed bits in that octect will be 0. Is that what you didn't understand?

jlhct · June 2008

dynamik wrote:

What did you expect it to be?

Write them both out in binary if you have trouble visualizing it. Both bits you're ANDing have to be a 1 in order to have the result be 1. Since the last octect is all 0s, all ANDed bits in that octect will be 0. Is that what you didn't understand?

I was expecting the 1st octect to be 145 instead of 146, since the 1st one should look like this:

10010001

If I add those up, i'd get 145 right? I think thats what I'm not getting.

thanks!

undomiel · June 2008

Ok, break out the calculator.

156.64.160.9 in binary:

10010010.01000000.10100000.00001001

255.255.224.0 in binary:

11111111.11111111.11100000.00000000

Then we'll do a bitwise AND on them.

10010010.01000000.10100000.00001001
AND
11111111.11111111.11100000.00000000
=
10010010.01000000.10100000.00000000

Which converts to you 146.62.160.0 number.

Let's do another one.

156.64.62.102
255.255.224.0

10010010.01000000.10100010.01100110
AND
11111111.11111111.11100000.00000000
=
10010010.01000000.10100000.00000000

You end up with the same number 146.62.160.0 so that means that 146.62.162.102 is in the same subnet as 146.62.160.9.
This is delving into subnetting. 146.62.xxx.xxx is normally a class B network. 255.255.0.0 mask. Using 255.255.224.0 borrows some of the bits from the hosts assignments to give additional subnets but not so many hosts per a subnet.

I'm not a subnetting expert by any means so I'd recommend taking a look at royal's post and learntosubnet.com for a much better explanation. Or someone else will post a much better explanation. But that should give you a start.

dynamik · June 2008

jlhct wrote:

I was expecting the 1st octect to be 145 instead of 146, since the 1st one should look like this:

10010001

If I add those up, i'd get 145 right? I think thats what I'm not getting.

thanks!

Well, that does add up to 145, but where did you get 145 from?

Also, keep these shortcuts in mind:

If one of the octets is 255, the value will be identical to the other octect. i.e. 255 AND 146 = 146
If one of the octets is 0, the value will be 0. i.e. 9 AND 0 = 0

The only octet you need to do any work on is the one that falls somewhere in the middle.

jlhct · June 2008

Ok, I think the book has it listed wrong.

They have 10010001 for the first octet (146)where I think it should be like undomiel posted: 10010010

This is why I'm only adding up to 145 instead of 146.

All of the rest of the #'s in the example add up.

I was starting to think I was too dense to pick this up LOL

I'll definitely check out that subnetting post, thanks!

dynamik · June 2008

You can register your book here to view the errata for it: http://www.syngress.com/solutions/

You should always have a print-out of the errata handy, so you don't drive yourself crazy because of their errors.

jlhct · June 2008

dynamik wrote:

You can register your book here to view the errata for it: http://www.syngress.com/solutions/

You should always have a print-out of the errata handy, so you don't drive yourself crazy because of their errors.

HAHAHA Thanks!!

I lost my passoword so i'm waiting for that before I proceed. Thanks for that link!

Bitwise Anding question

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