The subnet mask was the forerunner of the modern IP address prefix length. When configuring a host, both a 32-bit dotted-quad IP address and a 32-bit dotted-quad subnet mask was specified The subnet contains a one bit in every position where the prefix is valid and must match, and a zero bit in every remaining position, where the prefix is ignored. Since 255 is 11111111 (eight ones) in binary, a mask of 255.255.0.0 has sixteen one bits and corresponds exactly to a prefix length of /16.

So, a subnet mask of 255.255.255.0 conveys exactly the same information as a prefix length of /24.

A subnet mask of 255.255.255.240 is equivalent to a prefix length of /28.

It is important to master both forms, since the older mask syntax is quite common, particularly during system administration tasks, though the slash/prefix length style is gaining popularity.