Paul Gregg has shared a method he's come up with to determine if a given IP is within a selected range.

Unfortunately although people usually understand that an IP address is simply an unsigned 32 bit integer, and is easily determined, usually with $_SERVER['REMOTE_ADDR'], where the real challenge is - is in specifying the range within which they wish to check that IP address. IP ranges are usually specified in three common ways (in increasing complexity): wildcard, start-end range, classless inter-domain routing.

He looks at each method and includes descriptions and code examples as well as a link to the source code and a live demo of it in action.