## Easy to remember subnetting trick

01 Mar 2019

If you are related to the world of networking, you would of course have had to subnet at some point of your career. Though it is rare that you may have to do it without the help of the internet, I am willing to bet that at least most you would have had to do this at least once ( maybe in an interview? ).

Here is a neat trick that I like to use as a little cheat for subnetting without using a calculator.

What I do is, I consider the IP addresses as memory units, specifically megabytes. You dont even have to be from the world of computers to be familiar with these memory units.

Consider:

``````1 IP Address  = 1 MiB
``````

Generally in storage devices like flash drives, memory cards and so on, we see the storage capacity incrementing as exponents of 2. The same can be applied in subnetting.

With this I came up with the below table.

/32 1 MiB 1
/31 2 MiB 2
/30 4 MiB 4
/29 8 MiB 8
/28 16 MiB 16
/27 32 MiB 32
/26 64 MiB 64
/25 128 MiB 128
/24 256 MiB 256
/23 512 MiB 512
/22 1 GiB 1024 Convert GiB to MiB
/21 2 GiB 2048
/20 4 GiB 4096
/19 8 GiB 8192
/18 16 GiB 16384
/17 32 GiB 32768
/16 64 GiB 65536
/15 128 GiB 131072
/14 512 GiB 262144
/13 1 TiB 524288 Convert TiB to Mib
/12 2 TiB 1048576
/11 4 TiB 2097152
/10 8 TiB 4194304
/9 16 TiB 8388608
/8 32 TiB 16777216
/7 64 TiB 33554432
/6 128 TiB 67108864
/5 256 TiB 134217728
/4 512 TiB 268435456 Convert PiB to Mib
/3 1 PiB 536870912
/2 2 PiB 1073741824
/1 4 PiB 2147483648
/0 8 PiB 4294967296

In any given subnet (except /32 which is one IP address), subtract 2 from the number of IP addresses and you get the number of usable IP addresses ( 1 network address and 1 broadcast address).

I agree that this method is not particularly useful with the larger subnets like /8, but this makes it easy to calculate the smaller ones. Anything below /18 is a piece of cake.

On a side note, contrary to popular belief, 1024 MB (Megabyte ) is not equal to 1 GB (Gigabyte). 1000 MB = 1 GB.

The memory units used are Mebibyte (MiB) , Gibibyte (GiB) , Tebibyte (TiB) and Pebibyte (PiB).

``````1024 MiB = 1 GiB
1024 GiB = 1 TiB
1024 GiB = 1 PiB
``````

Hope this was useful.