Port-Restricted Cone NAT RouterYour internet connection is behind a Port-Restricted Cone NAT Router that may prevent you from connecting to some players in peer-peer games.
You will still be able to connect with the majority of players, but not with those who have Symmetric NAT Routers (which are not that common, fortunately). One advantage of fixing this problem is that the "Preparing Connection" window will complete much faster.
If you do want to solve this problem, it will involve you configuring your router. Most routers are configured through a web page built into them.
Many recent routers support a feature called Universal Plug and Play (UPnP). This feature will allow GameRanger to do most of the configuration required. You will need to enable it to allow GameRanger to make changes. Not all routers support UPnP correctly, however, so that may not be enough.
The alternative is to manually configure your router to do port-forwarding for UDP port 16000. Ask your ISP (Internet Service Provider) or refer to your router's instruction manual for assistance here.
Port-forwarding tells the router to send the game communication data to your computer. The port-forwarding configuration page will ask you for a public port and a private port. Set both of those to 16000. In some cases, instead of asking for a public port, it will ask for a port range (eg. Start Port, End Port). Set the Start and End to 16000. It will also ask you for the IP address of your computer that the data should be sent to. It may call this the LAN IP, Local IP, or Private IP. This should be set to 10.15.34.61. If it asks you for the protocol or type, set it to UDP.
Unfortunately, it is not possible to offer step-by-step instructions here as every router is different.
If you are at work on a company network or at a university, you may not be able to fix the problem yourself. Instead you may have to ask the network administrator to how to port-forward UDP port 16000 to your computer.