from
The Free On-line Dictionary of Computing (8 July 2008)
address mask
subnet mask
<networking> (Or "subnet mask") A {bit mask} used to identify
which bits in an {IP address} correspond to the {network
address} and {subnet} portions of the address. This mask is
often referred to as the subnet mask because the network
portion of the address can be determined by the {class}
inherent in an IP address. The address mask has ones in
positions corresponding to the network and subnet numbers and
zeros in the host number positions.
(1996-03-21)