Subnetting 1 2 3
By Adam Chee W.S
Ever get stressed out because you know that there would be subnetting question(s) in the
next exam you are taking and that these questions easily take up 10 to 20 minutes of your
precious exam time? What if there is more than one question?
The process of converting the subnet to binary and decimal can drive the unfamiliar insane,
not to mention the waste of precious time and brain power which can be utilized for other
areas of exam preparation.
Let's take a look at a shortcut method that will cut down the time needed to answer these
questions without the need for a calculator.
This article assumes that you know how to perform ...view middle of the document...
Its actually quite
Subnetting 1 2 3 080119.doc
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simple, lets look at it line by line.
1) Bits borrowed, this is the easy one, just remember that the table consists of only 8
2) Bits Value, remember by heart that the first value starts with 128 and the subsequent
values are divided by two.
3) Subnet Mask, this line tells you what the subnet mask would be, to get the figures, add
up the corresponding bits value and all of the values prior to it.
128 + 0 (there is no prior value) =128
128 + 64 = 192
192 + 32 = 224
224 + 16 = 240
240 + 8 = 248
248 + 4 = 252
252 + 2 + 254
254 + 1 + 255
4) Number of Subnets, tells you how many subnet you'll get if you use the subnet mask.
Just look at the corresponding N value at the top and you can derive the figures.
Once you understand how to derive the 'subnet table', spend some time practicing. I would
advise you to draw out the table once you are in the exam room (before starting the actual
exam) it will take you less than a minute.
How to tackle the questions
There are only a few different ways that Microsoft or Cisco can phrase their questions, lets
take a look at some examples,
Question Type 1:
If you are to determine the subnet mask based on a number of hosts and an IP address
You are assigned an IP address of 172.30.0.0 and you need 1000 hosts on your
network, what is your subnet mask.
Step one: Determine the number of bits needed for the hosts.
In this scenario, we need ten bits as 2^10 = 1024 (the question asks for 1000 hosts only)
Step two: Determine the number of bits left for the subnet.
32 - (number of bits needed for the host) which is 32-10 = 22 bits
Step three: Determine the number of bits actually borrowed.
We take the number of bits left for the subnet and minus as many 8s as possible as each 8
represents 1 octal. Therefore 22 - 8 - 8 = 6 bits were borrowed