Definition: STUN

(Session Traversal Utilities for NAT) An IETF protocol for real-time voice, video and messaging in an IP network. STUN provides the mechanism to communicate with users behind a network address translation (NAT) firewall, which keeps their IP addresses private within the local network (LAN).

The initiating party sends a request to the STUN server, which maintains the IP addresses of the phone or computer (for video). After that, a peer-to-peer connection is established using protocols such as WebRTC or ICE. Unlike application-layer gateways (ALGs), which also support two-way communications through NATs, STUN requires no configuration in the router.

Originally known as Simple Traversal of UDP through NAT, the name was changed in 2008 in order not to mislead people in thinking it is a complete solution to traversing the NAT.

TURN (Traversal Using Relays Around NAT)
For very restrictive corporate networks, a TURN server may be required, which serves as a relay. See NAT.

STUN and WebRTC

In this videoconferencing example, if the networks are not too restrictive, the STUN server enables a direct connection via WebRTC. Otherwise, a TURN server can be used. (Image courtesy of Interactive Powers LLC.)