ComTech: IT Support Stirling

Tutorial – How to Subnet

ComTech: IT Support Stirling

Today I will show you a technique I learnt for my Cisco exams on how to subnet and choosing subnet masks.

The best way to explain subnetting is by showing some examples.

Question 1:

If a subnet mask of were used with a Class A network how many subnets and hosts per subnet could exist?


The best way to answer this question is to break it down into individual sections.  The sections are:

1. No of Network bits

2. No of Host bits,

3. No of Subnet bits,

4. No of Subnets

5. Hosts per Subnet.

Lets take them one at a time.

No of Network bits = 8 (This is defined by Class A = 8, Class B = 16 and Class C = 24)

No of Host bits = 16 (This is defined by the number of zeros in the subnet mask)

No of Subnet bits = 8 (This is defined by 32 – No of network bits – No of Host bits)

No of Subnets =  256 (This is defined by  2ˆ No of subnet bits)

Hosts per Subnet = 65534 (This is defined by 2ˆNo of Host bits – 2)

So to answer the question  No of subnets = 256 with 65534 hosts per subnet.

Question 2:

Which of the following are valid subnet numbers in network when using mask




Again the best approach is to break the question down into sections.  This time the sections are:

1. Find the Subnet Number

2. Calculate the First address in the range

3. Calculate the Broadcast address

4. Calculate the last address in the range

Lets take them one at a time.

Subnet Number = (This is defined by 256 – 248 = 8 (Subnet Magic Number). 8 *0 = 0 (Interesting Octet) which is the closest multiple <=0 which is the 3rd octet in

First number in the address range = (This is defined by adding 1 to the subnet’s last octet)

Broadcast Address = (This is defined by the Subnet Magic Number (8) + Interesting Octet (0) minus 1

Last address in the range = (This is defined by the broadcast address -1)

So to answer the question the subnet numbers begin with (zero subnet) and then, and so on therefore a and c are the correct answers.

I hope that the steps outlined above help when it comes to subnetting your own networks.

About the Author


Hi I’m Chris Wakefield the owner of ComTech IT Support. I provide Windows, Mac and Linux based IT Support to small businesses in Stirling, Alloa and Falkirk.

Follow @Comtech247 on Twitter

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