Bitwise Anding question

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!

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dynamik

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

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

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

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

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

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

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!