A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address. For example, consider the IP address 150.215.017.009. Assuming this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on this network.
What is Subnetting?

Subnetting enables the network administrator to further divide the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. This is easier to see if we show the IP address in binary format.

10010110.11010111.00010001.00001001

The Class B network part is:

10010110.11010111

00010001.00001001

If this network is divided into 14 subnets, however, then the first 4 bits of the host address (0001) are reserved for identifying the subnet.

The subnet mask is the network address plus the bits reserved for identifying the subnetwork — by convention, the bits for the network address are all set to 1, though it would also work if the bits were set exactly as in the network address. In this case, therefore, the subnet mask would be 11111111.11111111.11110000.00000000. It’s called a mask because it can be used to identify the subnet to which an IP address belongs by performing a bitwise AND operation on the mask and the IP address. The result is the subnetwork address:

The subnet address, therefore, is 150.215.016.000.

Subnet Calculator

An IP subnet mask calculator is used to automatically calculate subnets. The calculator allows you to input an IP address and choose the Subnet Mask, Network class and other variables to calculate subnet network mask. Results of the calculation will provide the hexadecimal IP address, the wildcard mask, subnet ID, broadcast address and the subnet address range for the resulting subnet network.

There are a number of free online IP Subnet calculators including:

Online IP Subnet Calculator
Web Tools Subnet Mask Calculator Tool
ipcalc
Monitis Subnet Calculator

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