The router keeps track of the client ephemeral source port to map to which private IP it needs to send the return packet. If devices inside the same network use the same source port, destination IP and destination port then something like Dynamic NAT with port overload is used : What it will do is map the port with another ephemeral ports (usually +1) that’s free. The router keeps track of all those mappings.

There is a online map with all red mailboxes in Brussels. I guess that’s where you can get updated info about mailboxes. If you need to send important mail, always go to the post office to ask for acknowledgement of receipt or registered mail.

I’m not 100% sure, but I heard that you need to notify sibelga for new installation and if you want to inject to the grid you’ll need the smart meter. I don’t think there is a ban if you want to self consume all your solar production. I’m looking to install PV panels at the moment and I’m thinking about adding enough batteries to self consume in the evening.