How does subnet mask add up to 255?

A subnet mask is used in IP networking to divide an IP address into the network and host portions. It consists of 32 bits, typically represented in four octets. Each octet can range from 0 to 255, and when the bits are set to '1' (indicating the network part) and summed across the four octets, they can total up to 255 for each octet. For example, a subnet mask like 255.255.255.0 has three octets set to 255 (all bits '1') and one octet set to 0 (all bits '0'), summing to 255 for each active octet.