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Comment by nullify88 | Hacker News Reader

It does depend on the game. A P2P game like Call of Duty will have problems when played simultaneously on more than one console because the external port can not be shared.

What happens though is that another external port gets forwarded to 3074 instead.

3074 -> 3074

3075 -> 3074

3076 -> 3074 etc.

Its likely uPnP does this automatically as even uPnP won't be able to map multiple devices to the same external port but it does make the process convenient.

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That’s not how NAT works though. It uses random ports, and there are no collisions, unless you have thousands of Xboxes behind a NAT. Also, remember that CGNAT is a thing, with hundreds of households( with dozens of game consoles) all behind a single IP address. Essentially, the people talking about game consoles not working are wrong.

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.

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?

Yes, CGNAT is causing a lot of issues not being able to host multiplayer games, as I've experienced myself (with PC games, though server-client ones, not peer to peer ones).

Anyway, these days, if you don't have an IPv6 /56, you don't have a real Internet connection.

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