The Super Easy Subnetting Technique

Subnetting questions may frustrate you (especially if the exam is approaching fast) and thus you may feel little stressed.  Subnetting the traditional way into binary and decimal can be troublesome and a very time-consuming process. This will waste lots of your precious time and energy on the exam.  Here, I will show you some shortcut methods by which you will be able to answer the subnetting questions in a much shorter time span just as accurately as with the long drawn out method.  (Note: you will not be able to use a calculator on the exam, which makes this technique all the more valuable!)

Subnetting Basics

I will assume you know the basic concept of subnetting for this tutorial.  Below is a summary table of the typical IP ranges we have to work with in any given situation:

Class Range Subnet mask Host bit Subnet
Class A 1 – 126 255.0.0.0 24 8
Class B 128 – 191 255.255.0.0 16 16
Class C 192 – 223 255.255.255.0 8 24

Note: It is highly recommended that you learn this table if you want to use the shortcut methods below.

The Subnet Table

Bits Borrowed (N) 1 2 3 4 5 6 7 8
Bit Value 128 64 32 16 8 4 2 1
Subnet Mask 128 192 224 240 248 252 254 255
Number of Subnets ((2^N)-2) 0 2 6 14 30 62 126 254

Note: Study this table and remember it.  It will save you precious time on your exam.

Using the table above we can quickly calculate any subnetting question such as determining the number of subnets for an IP, the number of hosts for a subnet, and so on.  Please see our resources and study guides pages for many more examples of subnetting.