- incoming port can be opened or
- UPnP enabled.
Is this incoming port opened on the NAT device so that all incoming messages coming to that port are relayed to my host?
EDIT: More resources about the topic https://bitcoin.org/en/full-node#enabling-connections and https://bitcoin.org/en/full-node#network-configuration
The second resource says: "If Bitcoin Core can’t automatically configure your router to open port 8333, you will need to manually configure your router."
Is this automatic opening mentioned actually UPnP way?
And manually configuring the router actually bullet 1 mentioned above?