STUN

STUN (Session Traversal Utilities for NAT) is a protocol that allows a VoIP endpoint behind a NAT device to discover its public-facing IP address and port. This information is then embedded in SIP signaling so the remote party knows where to send media.

STUN works well for most NAT types but fails with symmetric NATs commonly found in large corporate networks. In those cases, TURN (Traversal Using Relays around NAT) relays the media through an intermediary server. ICE combines both STUN and TURN for maximum connectivity.