Subnet mask

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.

However, the special subnet mask 0.0.0.0 is valid. This subnet mask is a wildcard that accepts all subnets.

The subnet value will be logically ANDed with the subnet mask to determine the subnet.