why AND-ing?

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I find AND-ing a confusing concept in subnetting and I was wondering if it's
necessary or not. To find the subnet where a host address lives on, we
simply turn off all host bits and find the subnets as multiple of last
subnet bit. The subnet where our host lives on is the subnet where the host
range includes our host, right?

Example: Host address 10.17.2.14/18
Class A, so turning off all hosts bits will give us the subnets as multiple
of 64. That is, 10.17.0.0; 10.17.64.0 etc.
Since in 10.17.2.14 number 2 is in the range of the subnet 0 (0 to 64) then
the answer is: Our host lives on subnet 0: 10.17.0.0

new guy :)



Re: why AND-ing?
new guy wrote:
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I'd like to see what other people have to say about this - I can't fault
your logic. Doing a bitwise AND on the host address and subnet mask
would yield the same answer.

Only thing that I'd say is that for the purposes of the CCNA exam, you
must assume that you can't use subnet zero, is that still correct?

Cheers,

Chris.

Re: why AND-ing?

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Thanks for the answer!
I would also like to have more examples and find the answers in a logic,
simple way, without having to write anything down. Just by looking at the
address you know how many bits have been borrowed, and that's all you need
to automatically find the subnet (as a multiple of the last bit borrowed)
with the hosts range where the given host lives on.

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This is beside the point, however: The first subnet (subnet 0) has the same
address as the network and it's considered "unusable subnet". The last
subnet is also considered "unusable subnet" because contains the network
broadcast address. Therefore, if there is an exam question on calculating
the number of subnets, the formula 2^n - 2 is used, where n is the number of
borrowed bits from the host portion inorder to subnet the network. I have
not taken the exam yet though :)

new guy :)



Re: why AND-ing?
new guy wrote:
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This is why I like Usenet so much. Lots of people explaining things in
different ways. Sometimes just the way an explanation is phrased can
help you understand something a little better.

I already knew the above, but something about the way you explained it
helped it to fit in my head a little better :) Thanks for that!

--
Chris M.

Re: why AND-ing?

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You are welcome! It's not easy to find good explainations about subnetting
:)

new guy :)



Re: why AND-ing?
i use 256 - subnet mask = first subnet address

ex:

/18 = 255.255.192.0

256-192=64

first non zero subnet for 10.17.0.0/18 is 10.17.64.0 host addreses(64.1 ->
64.126)
2nd subnet is 10.17.128.0/18 host addresses (128.1 ->128.190)


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Re: why AND-ing?
On Mon, 5 Jun 2006, it was written:

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Yes, that is basically what AND-ing is, i.e  x AND 1 = x, x AND 0 = 0.

So, in your example, you have:
10.17.2.14/18 (10.17.2.14 netmask 255.255.192.0) or
00001010.00010001.00000010.00001110 AND
11111111.11111111.11000000.00000000
-----------------------------------
00001010.00010001.00000000.00000000 = 10.17.0.0

Doan

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Re: why AND-ing?

new guy wrote:
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a use of AND is for comps (not necessarily humans) to retain some bits
and set others to zero.
comps don't need to look at multiples as you are. Comps figure out what
subnet a host is on by looking at the subnet bits.

i'm quite sure they then use "Not XOR " (XNOR) to test if 2 bit
patterns are the same.


Re: why AND-ing?
This is great.

I have just started my CCNA studies and finally got the concept of
Subnetting last night.  I like re-reading stuff like this in order to
see how other people see it in order to expose myself to different ways
of coming to the same solution.

Regards,

MrBigglesworth

q_q_anonymous@yahoo.co.uk wrote:
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Re: why AND-ing?

MrBigglesworth wrote:
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glad to hear it, good luck


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