STUN

From BitWise DocuWiki

STUN stands for Simple Traversal of UDP over NATs and is a method whereby software running on a computer behind a router can find out its Internet IP, the port used internally and externally for a given connection, and other information. STUN does require a STUN server that can report information back to the computer making the STUN request.

BitWise uses the STUN protocol to facilitate UDP Punching in conjunction with voice chat. You can select which public STUN server you want to use in the Voice preferences.

For more detailed information about the STUN protocol, please see STUN (Wikipedia).