John Draper was hacking computers long before computers were even common place. Draper's hacking heyday was back in the early 1970's, when the largest computer network to which the general public had any access was the telephone system. At the time, telephones were managed by an automated system using specific analogue frequencies which could be exploited to make free long distance or even international calls.
It was called "Phreaking," and one of the most well-known Phreaking tools was a toy whistle that came in a box of Cap'n Crunch cereal. With this whistle, Draper created another popular Phreaking tool known as the Blue Box, a device that could produce many other tones used by the phone companies.