Netmask

A netmask is a 32-bit binary mask used to divide an IP address into subnets and specify the network's available hosts.

In a netmask, two of the possible addresses, represented as the final byte, are always pre-assigned and unavailable for custom assignment. For example, in 255.255.225.0, "0" is the assigned network address. In 255.255.255.255, the final "255" is the assigned broadcast address. These two values cannot be used for IP address assignment.

Below is an example of a netmask and an example of its binary conversion.

Counting out the bits in the binary conversion lets you determine the netmask length. Above is an example of a 32-bit address. However, this address is a broadcast address and does not allow any hosts (computers or other network devices) to connect to it.

A commonly used netmask is a 24-bit netmask, as seen below.

Using a 24-bit netmask, the network would be capable of 2,097,150 networks or 254 different hosts with an IP range of 192.0.1.x to 223.255.254.x, which is usually more than enough addresses for one network.

A simple formula can determine the capable amount of networks a netmask can support.

2^(netmask length - # of used segments) - 2

For example, if we used a netmask length of 24, having a netmask of 255.255.255.0 with three used segments, subtract three from the netmask length, e.g., 24-3 = 21. With this number determined, plug it into the above formula to get 2^²¹ - 2 = 2,097,150 total number of networks. You are subtracting two from this number because of the broadcast and network addresses that are already being used.

Another example is a netmask length of 16, having a netmask of 255.255.0.0 with two used segments. Using the above formula, you would get 2^¹⁴ - 2 = 16,382 total number of networks.

To determine the number of hosts a netmask can support, use the following formula.

2^(# of zeroes) - 2

For example, with a netmask length of 24, as shown in the above chart, there are eight zeroes. Therefore, using the formula above, this would be 2^⁸ - 2 = 254 total number of hosts. Again, two is subtracted from this number to account for the broadcast and network addresses.

Again, another example of a netmask length of 16, there would be 16 zeroes. This formula is 2^¹⁶ - 2 = 65,534 total number of hosts.

Below is a breakdown of each of the commonly used network classes.

Binary, IP address, Network terms, Subnet Mask