UDP, NAT and setting up “connections”

Question

I know the word \"connection\" isn\'t really appropriate when talking about UDP, but...

How does a server (the one with the known IP) get its UDP packets through the Int

Answer 1

Ignoring the provision of known port translation (i.e. data on this port goes to this address) in your router (providing the NAT), you can use UDP Hole Punching.

I assume you're not talking about multicasting, in which each peer joins a group and announces that to interested parties (in this case the router), which can then perform the appropriate routing. Although that's normally used for routing traffic efficiently to multiple hosts, the routing-by-group mechanism would work for you're describing above.