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In Port Address Translation, PC with Private IP address sends a packet with Destination address as a public IP address to the router. The router changes the Source address to the public router address and assigns a random port number for response. When packet reaches the destination, it should see return address and port. The destination sends a packet with routers public ip and same port. The router then remaps the packet to internal ip using the port number information. The port then stops listening after a timeout.

Websites like Whatismyip show only the routers public ip and not the port. How can I find the port number?

If the port number is found, can I use this as a temporary Port Forwarding? For example, send a packet to router public ip:port to connect to the computer. Or will the router ignore it because the source address was different compared to the destination address of the packet sent.

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I have not tried it myself, yet, but I wonder if Wireshark would display the IP:Port in any of the packets it captured.

  • The packet with the port will be between the router and destination. To use any packet analyzer, I would have to be on the public internet on one of the route the packet passes through. Is there a web service or a some other method that can tell me the port? – fanix May 25 '17 at 12:02
  • @fanix If you can run tcpdump, tshark or similar on your router, that would suffice. – user4556274 May 25 '17 at 12:16
  • I don't have access to the router that has a public IP. I have a WiFi router, which connects to my building's router which connects to ISP's router which has the public IP, I think. – fanix May 25 '17 at 13:14

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