What exactly is the function of a subnet mask?

A computer system in IPv4 networking will compare the subnet mask to the IP address to determine the network and host number of the IP address. A subnet mask is not used in IPv6 addresses. IPv6 addresses are made up of eight segments. The first three segments are a site prefix, the fourth is the subnet ID, and the final four are the interface ID, which is essentially the host address. Nothing is hidden by the subnet mask.