Simple Tricks To Ace
the Subnetting Portion
of Any Certification
Exam
1-800-COURSES
www.globalknowledge.com
Expert Reference Series of White Papers
Introduction
Subnetting seems to be a battle of fighting bits, decimal numbers, and countless methods and processes to
convert from one to the other. While the methods may be confusing, the mathematics behind them is the same
for all. In this paper, you will learn some of the simpler ways to figure out many of the subnetting questions
that you will find on the industry certification tests.
Unlik
e some of the more complex methods, these methods use subtraction,
addition, multiplication, and divi-
sion—no converting from binary or decimal. As a matter of fact, if you can do the four basic math functions,
you can learn these failure-free methods quickly and easily.
Warning: The basic assumption is that you are already familiar with subnetting and have actually learned
subnetting concepts elsewhere
. This white paper does not teach subnetting,
it teaches useful methods for
passing certification test questions.
Overview of Subnetting
The reason we subnet is to break larger IP networks into smaller ones. Often we have networks that are the
same size. These use a fixed length subnet mask for all networks. Other network designs employ different sub-
net masks, depending on the number of addresses required for each subnet. This is called variable length sub-
net masking or VLSM.
As I learned subnetting, I began to realize that subnetting is much like my grandmothers kitchen. When my
grandmother made pies, she cut the pies in various configurations depending on the needs of the pie eaters.
Often, the pie was cut with all pieces the same size. Other times she cut the slices in various sizes, depending
on who was eating. My grandfather always got the biggest piece . . . go figure.

Mask Table
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raining LLC. All rights reserved.
Page 3
Address Class First Octet Value Range First Octet Binary Value
A 0-127 0nnnnnnn
B 128-191 10nnnnnn
C 192-223 110nnnnnn
D 224-239 1111nnnn
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
128 64 32 16 8 4 2 1
Binary Mask Addr
esses
00000000 0 256
10000000 128 128

To convert from prefix to mask: rather than add, you will subtract.
In this example, you are asked to convert the mask prefix /23 to a mask. Here’s how it is done.
Begin by subtracting 8 from the prefix number. 23 – 8 = 15 255.
T
hen subtract another 8 from the remainder
15 – 8 = 7
255.255.
Find the seven bit entry in the mask table and add it to the mask 255.255.254.
Since there are no bits left, add 0 to the mask 255.255.254.0
The answer is 255.255.254.0
Another w
ay to arrive at the same solution is to tak
e the prefix and divide it by 8.
23 / 8 = 2 with a remainder of 7.
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raining LLC. All rights reserved.
Page 4
T
here are two 255s in the front of the mask with seven additional bits in the third octet and no bits in the
fourth. 255.255.254.0
I prefer the remainder method. Here’s another example.
Convert /29 to a subnet mask.
29 / 8 = 3 with a remainder of 5
There are three 255s in the mask with a five bit fourth octet. 255.255.255.248
What Mask To Use, Part 1
One of the problem classes used in certification tests is the “what mask” class. You are given a description of
a networking situation and are asked to select the correct mask to use in subnetting the network.
Here is a typical question:
XYZ Corporation is using the 192.168.100.0 private address to implement a workgroups in their network. Each
workgroup will consist of 17 devices requiring IP addresses. One additional address is required for the router