That's where the signaling comes in, the "caller" generates an "offer" and this has to be relayed to the other end(s). from there, the clients take that and ge

That's where the signaling comes in, the "caller" generates an "offer" and this has to be relayed to the other end(s). from there, the clients take that and generate an answer that links the two.... the offer / answers include all of the ip info the machine can generate, but on a nat network that's really just only as far as your local gateway, your ipv6 addresses, etc... you need a STUN server to add in the parts that would include your external ip... that's really all stun does is some service further up the network to give a more reachable address..