IAX is the Inter-Asterisk eXchange protocol. It is used primarily for VoIP, but supports any streaming media. It uses a single port for the control and media stream. It is currently an IETF Internet-draft (expires May 2006). For more information see the Voip-info.org Wiki or the Asterisk web page.
IAX version 2 defaults to UDP port 4569. IAX version 1 used UDP port 5036.