IPv6 is a new internet protocol that we didn’t mod Freelancer to support yet. In case you have a rather recent OS such as Vista and later, you want to disable it (google for as to how) in order to have your game use the same protocol as the servers do.

Also, keep in mind, that most servers only support clients that have their respective mod installed, so sometimes you want to filter the server list ingame to display only those with compatible builds so only those you can join are displayed.