An IP address is divided into two parts, the network ID and the host ID. The subnet mask "hides" the network ID part of the IP address, leaving the host ID part as the only identifier.
A subnet mask is made by setting the network ID bits to "1s" and the hosting ID bits to "0s". For most computers, will use the subnet mask 255.255.255.0, which is the default for a Class C IP address. However, large networks may also use Class B default subnet mask 255.255.0.0 or Class A subnet mask of 255.0.0.0.
To view your network's subnet mask on Windows, open Command Prompt (Windows key + R)> type: ifconfig /all and Windows will bring up a list with information for your network, including your subnet mask.
For Mac, open System Preferences > Networks > Advanced > TCP/IP and you'll find the subnet mask there.