A popular implementation of TCP/IP and associated protocols for amateur packet radio systems and personal computers connected via serial lines. It was named after the call-sign of Phil Karn - the radio ham who first wrote it for MS-DOS on the IBM PC.

KA9Q is currently maintained by Anthony Frost <vulch@kernow.demon.co.uk> (call-sign G8UDV) and Adam Goodfellow <tcpip2@comptech.demon.co.uk>. It has been ported to the Archimedes by Jonathan Naylor (G4KLX).