Are there public STUN servers?
A STUN server operates on the public network and responds to the common question – 'What is my IP address? ' Using the STUN server, clients can access information about their public address and the type of NAT running on the gateway.
Is Google STUN server free?
The Google STUN server is something you can freely use for development purposes, but, as a free service, there is no SLA.