NetComm 4-port and 8-port Gateways Gateway Series - Manual (Page 59)

SmartVoice Gateway(s) User Guide 59
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STUN Inquiry
Using “STUN Inquiry” to detect your IP sharing device’s NAT type and communication between STUN
server and client (built-in SmartVoice gateway).
NOTE: If detected NAT type is “Symmetric NAT”, then the gateway is not able
to traverse the NAT of your Internet sharing devices. This is not a flaw of the
gateway’s design, but nature of STUN protocol design.