International server ?

I made a game where the networking is based on Larus sample.
It works well on local network of OSX machines, one instance of the program can create a server, the server is seen and can be connected by other instances.

When on heterogenous machines located in US and Canada however it doesn't quite work:
An instance can create a server and server is visible by another instance - just the 2 first lines though and it cannot be connected to.

Any idea?