A STUN (Session Traversal Utilities for NAT) server is a software program that runs on a server or other computing device and answers incoming requests. The answers that it sends out also include the public IP address from which the request was sent. Therefore, a STUN server automatically tells you what your public IP address is.
With a STUN server, NAT clients can set up phone calls to a VoIP provider that is hosted outside of the local network. Clients can also find out the type of NAT they are behind. In addition, they can discover the internet side port associated by the NAT with a particular local port. With this information, you can set up UDP communication between the VoIP provider and the client and make calls.
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