Use to specify the subnet mask (in dotted decimal notation) of the subnet this interface attaches to; 255.255.192.0, for example.
Rules:
The 32-bit subnet mask must be one or more one-bits followed by one or more zero-bits. The subnet mask cannot have any one-bits to the right of any zero-bits. Therefore, a mask of 255.255.192.0 is valid because 255 is 11111111 and 192 is 11000000, but a mask of 255.255.208.0 is not valid because 208 is 11010000.
Entering the subnet mask is optional. If left blank, a subnet mask is generated based on the class of the IP address.
Class A IP addresses begin with a decimal number less than 128. Consequently, a subnet mask of 255.0.0.0 is generated.
Class B IP addresses begin with a decimal number greater than 127 and less than 192. Consequently, a subnet mask of 255.255.0.0 is generated.
Class C IP addresses begin with a decimal number greater than 192. Consequently, a subnet mask of 255.255.255.0 is generated.