Calculating Total Number of Subnets Per Class

For our real life communication, we generally use Class A or Class B or Class C addresses. And the ISPs or the network engineers are always working on these classes of IP addresses. In the tutorial Classes of IP Address, we already discussed that Class D and Class E addresses are not generally used and they are used for Multicast and experimental /research.

In the following table a summarized scenario is represented where you will see the network part and host part of Class A, Class B and Class C IP addresses. Net means the Network part of that class IP address and H means the Host bit of that Class IP addresses.

Class	Total Number of Subnets Per Class	Subnets
Class A	01111111.00000000.00000000.00000000	Net.H.H.H
Class B	10111111.11111111.00000000.00000000	Net.Net.H.H
Class C	11011111.11111111.11111111.00000000	Net.Net.Net.H

The red colored bits are the most significant bits of each class of IP addresses. For details, read the tutorial IP Address Classification. So, when we would like to think about the no of subnets those bits are not needed to think.

Our goal is to determine the number of subnets of each class. In the table, you will find some bits are marked by yellow color. To calculate the total number of subnets we need to think the total number of those yellow colored bits because those are the network bits of that IP addresses. So, the total number of subnets can be calculated by using the equation = ( 2^{yellow_colored_network_bits} – 2 )

Class	No of bits to count	No of Subnets
Class A	7	27 – 2 = 126
Class B	14	214 – 2 = 16,384
Class C	21	221 – 2 = 2,097,152