NAT Mapping Behavior Test

Opens one local UDP socket and asks several STUN servers what public address:port they see. Same port everywhere → endpoint-independent mapping, STUN suffices for hole punching. Different ports → address(-and-port)-dependent mapping ("symmetric NAT"), TURN needed against another hard NAT.

Runs entirely in your browser via RTCPeerConnection — no data leaves your machine except STUN binding requests. Disable VPNs / iCloud Private Relay for a clean read of your router. Reflexive candidates missing entirely usually means UDP is blocked.