Suppose that two computers use the same Wi-Fi to access the Internet. Each of these computers has the same program installed, which is bound to the same UDP port. I want to know, since both computers have the same external IP address and listen to the same port but on different machines, what will be the result if a UDP datagram is sent to this common external address and to a common port, then which machine will receive it and how to send it each machine its own personal datagram?

CodePudding user response：

The router will not forward the packet to either computer, since it doesn't know which one it should forward to.

In fact, even if the program was only running on one computer, the router still wouldn't forward the packet. It has to see outbound traffic going from the computer to the outside world first, before it decides which external port to use for forwarding inbound traffic back to the computer. And the router might not decide to use the same port on the public IP that the computer used on the private IP.

This is why everyone hates NAT and likes IPv6.