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I could use some help with my way of calculating usable subnets and usable hosts.

the way i normally do it: (class B)

network | host
169.199 | 109.137
255.255 | 255.192 = 11111111.11000000 = 10(1) 6(0)

2^10 = 1024 usable subnets
2^6 = 64-2 = 62 usable hosts

that is the way I normally do it but my teacher would like to me use another method since i have use my own method wrong on multiple occasions.

His version: (class C)

network bits (n), subnet bits (s) and host bits (h)
then from that he gets the numbers he needs above the 2^?

2^? = ? usable subnets
2^? = ?-2 = ? usable hosts

I don't understand how his way works, how he gets the correct amount of n, s and h, and what to place above the 2^

I am one week into learning about ip's and subnetting, so when you give me a way to learn his way do explain it so I would understand it.

marked as duplicate by Ron Maupin, YLearn Mar 18 '16 at 15:32

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  • 2
    You should mention to your teacher that classful addressing is obsolete, and has been since before you were born. It's really time they stop teaching it. – Ron Trunk Mar 18 '16 at 11:45
  • 1
    I was about to say the same thing... I don't understand why it's still taught this way (even if there's some cases were you need to know what classfull addressing is like in Microsoft Windows VPN client settings where by default a "class based route" is automatically added) – JFL Mar 18 '16 at 11:59
  • This is really a duplicate question. See this answer for the correct way to do this: networkengineering.stackexchange.com/a/7117/8499 – Ron Maupin Mar 18 '16 at 13:53

Your teachers way and your way are exactly the same, just written down differently. To use your way as the teachers way, replace the 1's you wrote down with s and the 0's with h. So:

network | host
169.199 | 109.137
255.255 | 255.192 = 11111111.11000000 =       10(1) 6(0)
nnnnnnnn.nnnnnnnn | ssssssss.sshhhhhh = 16(n) 10(s) 6(h)

2^s = 1024 usable subnets 2^h = 64-2 = 62 usable hosts

  • Thank you very much it was just what i needed to understand it – Skovdam Mar 18 '16 at 9:13

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