STUN is is a protocol for assisting devices behind a NAT firewall or router with their packet routing. STUN allows applications to discover the presence and types of NATs and firewalls between them and the public Internet. It provides applications with the ability to determine the public Internet Protocol (IP) addresses allocated to them by the NAT. STUN works with many existing NATs and allows for a wide variety of applications to work through existing NAT infrastructure.
[Reference: Adapted from STUN RFC 3489]