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 **255.255.0.0** were used with a** Class A** network how many subnets and hosts per subnet could exist?

**Answer:**

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 **180.1.0.0** when using mask **255.255.248.0**?

a) 180.1.8.0

b) 180.1.4.0

c) 180.1.40.0

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** = 180.1.0.0 (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 180.1.0.0)

**First number in the address range** = 180.1.0.1 (This is defined by adding 1 to the subnet’s last octet)

**Broadcast Address** = 180.1.7.255 (This is defined by the Subnet Magic Number **(8) **+ Interesting Octet** (0)** minus 1

**Last address in the range** = 180.1.7.254 (This is defined by the broadcast address -1)

So to answer the question the subnet numbers begin with 180.1.0.0 (zero subnet) and then 180.1.8.0, 180.1.16.0 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.

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