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)