Subnet mask default

Specify one, two, three, or four segments of the subnet mask in decimal notation. This value will save keystrokes, prefilling subnet mask fields when defining links on subsequent customization panels.

For example you may enter all or a portion of the subnet mask as:
255.255.255.0     or     255.255.255     or     255.255     or     255

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.