Source : Free On-Line Dictionary of Computing

netmask
     
         A 32-bit {bit mask} which shows how an {Internet
        address} is to be divided into network, {subnet} and host
        parts.  The netmask has ones in the bit positions in the 32-bit address
     which are to be used for the network and subnet
        parts, and zeros for the host part.  The mask should contain
        at least the standard network portion (as determined by the
        address's {class}), and the subnet field should be contiguous
        with the network portion.
     
        If a `+' (plus sign) is given for the netmask value, then the
        network number is looked up in the {NIS} netmasks.byaddr map
        (or in the /etc/netmasks) file if not running the NIS service.
     
        {Unix manual page}: ifconfig(8).
     
        (1995-01-30)