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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, "0" is the assigned network address. In, "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 - 223.255.254.x. This is commonly plenty of 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 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 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
Class B 16 16,382 65,534
Class C 24 2,097,150 254

Also see: Binary, IP address, Network terms, Subnet Mask