So for the last few weeks I’ve been trying to learn how to do Subnetting off the top of my head. I’ve been using a great guide written by user LordFlasheart on the techexams.net forums. Here I’m going to explain how I interpreted the guide and am using it to subnet in my head…

I’m not going to explain the concepts of Subnetting here, only the operations that I perform in my mind when calculating the Subnet, Broadcast Address, and Host Range of a given IP.

1.**Determine the next largest subnet block from the prefix**

`blocks=[`

**8,16,24,31**]

**24** > /21, therefore **24** will be the block we’re working with.

24 is the 3rd octet (192.168.73.33)

2.**Calculate 2 ^ (block – prefix):** This gives you the block size

2 ^ (24 – /21) = 2^3 = 8 // 8 is our block size

3.**Determine the subnet address:** Octetx-(oxtetx%blocksize) = subnet address.

Octetx = 3rd Octet = 192.168.**73**.33 = 73

73 – (73*8) = 73-1 = 72

4.**Any octets after the one we’re working with = 0**

192.168.72.0

Tada, that’s it. The Subnet ID of 192.168.73.33/21 is 192.168.72.0

1.**Determine the next largest subnet block from the prefix**

24 > /21, therefore 24 will be the block we’re working with.

24 is the 3rd octet

2.**Calculate 2 ^ (block – prefix)** – This gives you the block size

2 ^ (24 – /21) = 2^3 = 8 // 8 is our block size

3.**Determine the next subnet’s address** – Octetx-(oxtetx%blocksize)+8 = next subnet’s address.

73 – (73*8) + 8 = (73-1) + 8 = 72 + 8 = 80

4.**Any octets after the one we’re working with = 0**

192.168.80.0

7. **Take 1 from that IP Address.**

192.168.80.0 – 1 = 192.168.79.255

Tada, that’s it. The Broadcast Address of 192.168.73.33/21 is 192.168.79.255

Host Range = (Subnet ID + 1) to (Broadcsat Address – 1)

Host Range = (192.168.72.0 + 1) to (192.168.79.255 – 1)

Host Range = 192.168.72.1 to 192.168.79.254