A subnet mask (or a dotted-decimal mask) is a 32-bit number used to identify the network part (or prefix) of an IPv4 address.
For example, IP 192.168.100.0 and subnet mask 255.255.255.0 is equivalent to 192.168.100.0/24 (indicating the first 24 bits are 1). The subnet mask means the first 24 bits are for network identification.