The Absolute Easiest Subnetting Method on Earth

Subnetting.... I still hate it, but now I don't fear it, NOW IT'S EASY. I remember when I just started learning subnetting and I saw all these methods being touted by each person as the best or easiest method when they were in fact quite complicated and often involving binary math.






The method I'm about to show you involves no binary math at all. All you need to do is memorize a simple chart.

It looks like this:

Class A = 1-126
Class B = 128-191
Class C = 192-223


Mask means all possible subnet mask octet values
Bits means number of subnet or "borrowed" bits
MN means magic number which is the increment between subnets or the number of addresses in the subnet
Snets means number if subnets (I'll explain this one in more detail later)

So if you are asked to find the subnet in which the IP address 172.16.44.2 /19 resides all you have to do is the following:

Just by looking at the IP address you can see that it is a class B address because it starts with 172, so that means it has 16 network bits (the first two octets). Since the mask is /19 just subtract 16 from 19 and you get the number of borrowed bits (3 in this case). Looking at the chart: 3 in the "bits" column matches with 32 in the "MN" column so compare 32 to the third octet in the IP address (this is the octet from which the bits were borrowed) the subnet number will be a multiple of the Magic Number. 32 goes into 44 once and if you multiply 32x2 you get 64 which is too much. So your subnet number will be 172.16.32.0

THAT'S IT - YOUR'E DONE!

For questions that ask you what mask will give you X number of subnets just look at the "Snets" colum in the chart and whichever value matches the number of subnets you need use the corresponding mask value in the "mask" column as the mask value needed to achieve that number of subnets. You can extend the chart to include more subnets by just doubling the figures as you go along.

Memorize the chart and practice the method over and over and it will become second nature to you. Just write down the chart before you start your exam and use it as a reference chart while doing subnetting questions on the exam.

Subnetting is a complex concept to grasp when you're just starting out so I may be posting more information on subnetting periodically and certainly on VLSM subnetting in the near future so stay tuned.

I hope this helps and I wish you all the best.


Roger.