STUN is an acronym for Simple traversal of UDP over NATs. It defines a mechanism that allows UDP based applications within a network using private IP addresses to learn their external IP address and port so that they can communicate with peers which reside within the public internet (and thus cannot send IP packets directly to a private IP address). The STUN server may be implemented by the IP router connecting the private network to the public internet (innovaphone devices have a STUN server from v11r1) or by a STUN server in the internet.