I guess, this is how it should be done 1. You get back your mapped port and mapped address from the STUN 2. CLient2 also gets back its mapped port and mapped address from the STUN 3. The STUN server should store these info. When either clients want to communicate with the other, it sends a request to the STUN server. The Stun server then sends a message to both the clients with the mapped Addr and port of their counterpart. 4. After this, hole punching can be done by the method shown in the article above.