How to Calculate the Number of Hosts Per Subnet

Subnets are part of an IP network.

In computer networking, subnets are used for routing purposes. Internet protocol (IP) networks are sometimes divided into logical subnets, which are parts of the network that can be distinctly addressed. When figuring out the number of hosts per subnet, you are limited by the number of bits that form the host ID according to the following mathematical equation: number of hosts per subnet = (2^H) - 2, where H is the number of bits in the host ID.

Instructions

1. Determine the number of bits that form the host ID. For example, you might have a host ID of 001 (3 bits).

2. Figure out what the number in Step 1 is to the power of 2. Powers of 2 are 2 raised to the nth power. For example, 2^2 = 4 and 2^5 = 32. In the above example, there are 3 bits in the host ID so 2 is raised to the 3rd power: 2^3 = 8.

3. Subtract 2 from the answer you calculated in Step 2. In the given example, the number of hosts per subnet is 8 - 2 = 6.

Tips & Warnings

• Powers of two multiply quickly (for example, 2^256 is 1.15792089 x 10^77). If you find yourself working with an impossibly huge number, double-check that you have counted the number of host digits correctly.

References

• BSoft: Calculating the Number of Subnets