KA9Q, also called KA9Q NOS or simply NOS, was a popular early implementation of TCP/IP and associated protocols for amateur packet radio systems and smaller personal computers connected via serial lines. It was named after the amateur radio callsign of Phil Karn, who first wrote the software for a CP/M system and then ported it to DOS on the IBM PC. As the KA9Q code was open-source, many radio amateurs modified it, so many different versions were available at the same time.