Disclosed herein is a network address translation (NAT) traversal method. A node sends a first, second, and third detection message from a local port to a first, second, and third response server, respectively, in order to receive from the response servers a first, second, and third response message, which respectively include a first, second, and third proximal external port number (EPN). The node then deduces a rule for proximal EPNs based on the received ones. Also disclosed is a system for mediating connection. The system assists a first node on a network in NAT traversal and includes at least three response servers, which receive detection messages from the first node and send back response messages that include EPNs.