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.
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