Calculating subnet masks
The IP Subnet Mask Calculator tool calculates the network address, the host address and the broadcast address for a given TCP/IP address and subnet mask. It will also tell you the number of available addresses in the network, displaying the first, last, and next addresses given the parameters entered.
Prerequisite(s):  
Classic mode must be enabled.
 
To use the IP Subnet Mask Calculator tool:
On the Classic tab, in the Tools group, click IP Calculator.
 
Only the top of the dialog is editable; the rest of the fields are determined by what you select in the first three controls. After making any changes, click Calculate to see the results. Click close when you are done.
IP Address: Enter the IP address for which you want to calculate subnet parameters.
Subnet Mask: Select the subnet mask for the network you are calculating parameters for. Depending on whether you have selected Show all masks or Show class-specific masks, the number of masks available on the drop-down menu will change.
Show class-specific masks: This choice lets you limit the mask selection drop-down menu to show only those masks valid for the current class of address. The first octet of the IP address defines the address class.
Show all masks: This choice expands the mask selection drop-down menu to include all subnet masks, including those masks that are not compatible with the current class. Address class is defined by the first octet of the IP address.