Network mask
A network mask is a mask that gets applied to an IP address. These are normally of the form 0.0.0.0 (empty mask), 255.0.0.0 (use the first 8 bits of the address), 255.255.0.0 (first 16 bits), 255.255.255.0 (first 24 bits), or 255.255.255.255 to use the whole address.