The subnet mask is a 32-bit binary number used to identify the network and subnet of an IP address. It is used to determine which portion of an IP address is the network and which portion is the host.
For example, the subnet mask for a Class C IP address is 255.255.255.0. This means that the first three octets (24 bits) of the IP address are used for the network portion and the last octet (8 bits) is used for the host portion.