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That is how NAT works for connections opened from private network to the Internet. If the console needs to listen to a port, the NAT must be configured to forward the listened port to correct device. Multiple devices behind NAT can not listen to the same port, and this is where the forwarding of different ports on public IP to same port on different private IPs comes in.

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The source port would be random and conmunication would ride on that established connection. An exposed port is not necessary for client initiated communication.

Yes, you're talking about a device behind NAT which initiates a connection to an external service.

However, we're talking about a service listening behind NAT (in this case Call of Duty on an Xbox at home) that needs to be listening for connections initiated by other Xboxes to establish a P2P connection. This is what port forwarding enables and this is what uPnP automates.

Why wouldn't they use a solution like https://samy.pl/pwnat/ this is a long solved problem that doesn't require uPnP.

Is it somehow better than uPnP?

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