Did you see my "Question of the Day" above?
How do the NTP response packets (UDP) from the server in Atlanta, or the DNS response packets (UDP) from the server in Fort Walton, get through my router back to my Arduino on a localnet ip, through my firewall and NAT without the routers communicating with each other?
Your router does the same. There is no connection. Unlike TCP, the UDP response packet is not associated with the request packet in any way as far as the enroute routers are concerned. They can't tell a request from a response with UDP. How does it know that particular port on my router's public ip is associated with the UDP port on my Arduino localnet ip to pass that response packet back? Magic pixie dust? Salted peanuts?
I'm not talking about source to destination two way communication like a TCP connection, just router to router as they pass the packet towards the destination ip.
As I said before, if you think this isn't it, please enlighten me on how it works. It would be a good thing for me to know.