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).