if you have a firewall, ensure it allows outbound UDP (and associated return traffic) to ports 1024..65535 inclusive.
if you have a NAT, make sure it isn't a symmetric NAT, or isn't set to symmetric mode. symmetric NATs are inherently defective* and should be reconfigured (if possible) or replaced. your NAT is probably symmetric if, when you go to
you get "No" for either of "source IP address is preserved from original connection" or "source UDP port number is preserved from original connection".
if you want to talk to other computers on your own network, don't use additional NATs behind a first NAT.
Sorry, I should have been more clear.
I understand that the router/firewall/network may prevent the connection. Assume that I do not have access to these, ie. I'm trying to set up p2p as best as I can for users of our website - not myself.