那甚麼是 STUN SERVER？
轉貼一個介紹：( 來源： http://www.3cx.com/PBX/stun-server.html）
What is a STUN Server?
A STUN (Simple Traversal of User Datagram Protocol [UDP] Through Network Address Translators [NATs]) server allows NAT clients (i.e. computers behind a firewall) to setup phone calls to a VOIP provider hosted outside of the local network.
所以我們的SIP 電話在防火牆裡頭、在NAT裡頭，外面是看不見裡頭的Device的，就必須要靠STUN SERVER來轉介。
The STUN server allows clients to find out their public address, the type of NAT they are behind and the internet side port associated by the NAT with a particular local port. This information is used to set up UDP communication between the client and the VOIP provider and so establish a call. The STUN protocol is defined in RFC 3489.
The STUN server is contacted on UDP port 3478, however the server will hint clients to perform tests on alternate IP and port number too (STUN servers have two IP addresses). The RFC states that this port and IP are arbitrary.
預設的 UDP port 是3478