Yes, you are right. The fastest way is to use an IP subnet calculator. You can find tons of free IP subnet calculators over the Internet, example like http://www.subnet-calculator.com/

But, what if something happened to your PC, where you can’t use the software or internet to do the calculation for you. Therefore, you should always have a backup, like every IT staff did. Make yourself a backup and learn how to count without any 3rd party tools.

Actually is not as difficult as you think. Here, I will show you an easy way to do it.

Given a scenario, if provides you the IP address and subnet mask. And ask you to find out what is the number of valid hosts, subnet and broadcast address.

Example:

IP – 192.168.10.34

subnet mask – 255.255.255.224 or CIDR (Classless Inter-Domain Routing) :/27

To find subnet and broadcast address.

1) 256 – 224(subnet mask last octet) = 32

First subnet is 32.

Second subnet is 32 x 2 = 64

Third subnet is 32 x 3 = 96

..

 

Since our IP (192.168.10.34) is fall between 32 – 64 subnet, so the subnet address is 192.168.10.32 and the broadcast address is 192.168.10.63.

Find Host range

1) Valid hosts range is 192.168.10.33 – 192.168.10.62

- as 192.168.10.32 is subnet, 192.168.10.63 is broadcast

Find total valid hosts

1) 32 – 27(CIDR) = 5

2^y = 2⁵ – 2

= 30 – 2

= 28

 

All solution only got 1 step. Simple isn’t it. Still don’t understand? Nevermind, let’s do another example.

202.188.0.50

255.255.255.240 or CIDR = /28

Subnet size: 256-240 = 16

Subnets are : 203.199.10.0, 202.188.0.16, 202.188.0.32, 202.188.0.48, 202.188.0.64

Our IP (.50) fall between 202.188.0.48 and 202.188.0.64 subnet.

Thus, our subnet is 202.188.0.48.

our next subnet is 202.188.0.64,

Thus, our braodcast address is 202.188.0.63

I assume everyone knows we can’t assign the subnet and broadcast address to any hosts, our valid hosts are between 202.188.0.49 – 202.188.0.62

32 – 28

= 4

2⁴ – 2 = [(2 x 2 x 2 x 2) - 2] = 14

Easy isn’t it.

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