A bitwise mask that shows which part of the IP address is shared by all computers in the subnetwork and is used to identify them all together in some larger network. The IP address part that is not flagged by this mask identifies individual computers in the subnetwork.
Netmask can be specified either in quad-dotted decimal representation like 255.255.254.0 (means the seven lower bits identify computers in subnetwork) or, together with IP address, as a number of the high bits used by the larger network and not by subnetwork (192.168.80.0/23, the same).
The bitwise AND operation between mask and IP address results the address of the network itself, and the remainder identifies the computer LAN connector within that network.