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Find available IP addresses given a subnet mask?

Posted on 2014-12-21
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Last Modified: 2014-12-23
Can someone help me learn what I formulas/conversion to figure out, if given the subnetmask of 255.255.255.224, what IP address would be available and on the same subnet?

This is an example question and I have been given the following 4 answers to choose 2 from:

10.4.4.125
10.4.4.158
10.4.4.165
10.4.4.189
10.4.4.199

It's a practice not a homework assignment or a test heh :)

Thanks
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Question by:garryshape
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Expert Comment

by:Chris Millard
ID: 40512216
There is an online subnet calculator that you can use at http://www.subnet-calculator.com
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Author Comment

by:garryshape
ID: 40512298
Thanks do you know one where it could show me how to break down the logical process instead of just giving answer?
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Accepted Solution

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Pete Long earned 1000 total points
ID: 40512529
Online calculators are great - till your in an exam :)

In your example....

255.255.255.224

(To use my method remember one thing - 256 is the magic number, thats it all you need to know)

So take the last octet from the magic number,

256-224=32

So each network is 32 addresses wide :) remember the lowest address in the network is the network address and the highest address is the broadcast address, to work out the subnets just start at zero and keep adding 32 e.g

0
32
64
96
etc etc

So network 1 is;
network address x.x.x.0
first usable addres x.x.x.1
last usable address x.x.x.30
broadcast address x.x.x.31

Using this method you can pretty much nail any cisco question for the CCNA sylabus - even questions like (you need a network with 6 hosts (so you know you will need 8 addresses (broadcast and network as well remember) 8 from 256 is 248 so subnet is 255.255.255.248 - Boom your done :)

Ive got a IP networking crib sheet here if you want it

Ive also got an online V4 and v6 IP calculator for when your not in an exam.

Pete
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LVL 26

Assisted Solution

by:Fred Marshall
Fred Marshall earned 1000 total points
ID: 40513325
Actually there are 5 entries that you call "answers".
From my reading of the question (which is at least a little ambiguous), the answers (plural) would be:
1 (is available on 10.4.4.96)
2 (is available on 10.4.4.128)
3,4 (are available on 10.4.4.160) <<<< is this what the question is supposed to be asking about???
5 (is available on 10.4.4.192)

As far as formulas/methods go, Pete Long gave that answer for a case like this one.
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Author Comment

by:garryshape
ID: 40515723
Thanks for the advice here, it really helps.

And this practice question actually popped up for a Comptia Security + one, which I found interesting.
It seems every IT exam these days that has to do with Windows is including Subnetting questions. I failed the MCSA Server 2012 because half the questions were on subnetting and IPv6 which 2 months of reading a study book (that had great reviews) provided me no information on.
So then I try to go back to something more basic like Security+ and it appears there's going to be subnetting here as well. Not sure on IPv6 stuff though (yet).
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