Netmask

Updated: 04/26/2017 by Computer Hope

NetmaskA netmask is a 32-bit mask used to divide an IP address into subnets and specify the network's available hosts. In a netmask, two bits are always automatically assigned. For example, in 255.255.225.0, "0" is the assigned network address. In 255.255.255.255, "255" is the assigned broadcast address. The 0 and 255 are always assigned and cannot be used.

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

Netmask: 255. 255. 255. 255
Binary: 11111111 11111111 11111111 11111111
Netmask length 8 16 24 32

Counting out the bits in the binary conversion allows you to 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 be connected to it.

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

Netmask: 255. 255. 255. 0
Binary: 11111111 11111111 11111111 00000000
Netmask length 8 16 24 --

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 be used to 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 3 used segments, subtract 3 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 2 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 2 used segments. Using the above formula, you would get 2^14 - 2 = 16,382 total number of networks.

To determine the amount of hosts a netmask is capable of supporting, use the following formula.

2^(# of zeroes) - 2

For example, with a netmask length of 24, as shown in the above chart, there are 8 zeroes. Therefore, using the formula above, this would be 2^8 - 2 = 254 total number of hosts. Again, 2 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. The formula in this case would be 2^16 - 2 = 65,534 total number of hosts.

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

Class Netmask length # of networks # of hosts Netmask
Class A 8 126 16,777,214 255.0.0.0
Class B 16 16,382 65,534 255.255.0.0
Class C 24 2,097,150 254 255.255.255.0

Binary, IP address, Network terms, Subnet Mask