Subnet mask calculation

5 pts.
Tags:
Microsoft Windows XP
Subnet mask
Subnets
Which host are in the subnet mask 172.16.16.0/20 and how do you confirm this to be applicable in other examples? 172.16.16.1/20 172.16.18.5/20 172.16.31.254/20 172.16.24.0/20

Software/Hardware used:
windows xp,vista,7

Answer Wiki

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My previous answer is wrong.
/20 subnets increment is by 16, not by 6 as I stated before.
Therefore, all addresses in your question reside in the same subnet.

P.S.: How can I edit my previous post and putting in it, strikeout text?

———————————————————————————————

Hello TB3,

Although when you already are proficient in sub-netting, explaining it here in a meaningful way is not that easy.
It takes some math and most of all practice…

The answer for your question is:

172.16.16.1/20
172.16.18.5/20

These two addresses are in the same subnet of 172.16.16.0/20.
Because the subnet mask is /20, every subnet increases in 6. So, being 16 (3rd octet) the first subnet, the next one will be 172.16.22.0/20.

You can read more about subnets, <a href=”http://www.akadia.com/services/ip_routing_on_subnets.html”>here</a>.

————————-

If you work the subnet bits you find this:

172.16.1111 0000.0000 0000 (the Class B /16 +4 = /20)
- subnet 1: 172.16.0.0 – 172.16.15.255 (hosts 0.1 – 15.254)
-<b> subnet 2: 172.16.16.0 – 172.16.31.255 (hosts 16.1 – 31.254) </b>
- Subnet 3: 172.16.32.0 – 172.16.47.255 (hosts 32.1 – 47.254)
— etc –

So – to answer your question, ALL of the Listed IP addresses are in the same subnet

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  • JOakey
    Sorry but the increment on a /20 is not 6 class C subnets but 16 class C subnets worth of space. A router wildcard mask for a /20 would be 0.0.15.255. The next block would begin at 172.16.32.0/20. There are websites that have good subnet calculators. I personally like this one - http://www.subnet-calculator.com/cidr.php. To answer part of the original question, to be good at subnetting you have to convert things to binary and then back to decimal or memorize the tricks around how the decimal increments relate to the binary bitwise ordering- ie /20 is 4 bits off of /24 so, therefore halfway between /16 and /24 so in binary that mask is 00001111. that's 15 but binary indexes at 0 so that is how you can quickly know it is segments into 16 Class C increments (0-15 is 16 subnets). Every IP address you listed in your original question falls within the 172.16.16.0/20 subnet.
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  • saturno
    Hello all, TB3 and JOakey I'm very sorry for my incorrectness. @JOakey is completely right. A /20 network increments are in 16. This will put all addresses you refer in the same subnet. Sorry for my lack of precision. I'll edit my first answer in order to prevent anyone to read an incorrect answer.
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