Every network device has a large number of ports by which they can send and receive information over the Internet. On Arelith, the servers currently listen for connections to ports 5122 (Cordor and Planes), 5123 (Surface), 5124 (Underdark) and 5121 (Distant Shores).
As well as these ports on the server side, your game uses its own port to connect. This article describes a port-related problem that some people may experience.
Often more than one person from a given network may want to play Neverwinter Nights at the same time. The problem arises when both are using the same client port. When two players connect to the same server from the same IP address using the same port, many problems will start to arise including missed Tells, server instability, "clones" appearing at random and failure to properly shut down during a reset. This is a well known bug in the Neverwinter Nights server program.
To protect our servers from these problems, Arelith has a check in place which causes them to ignore any new connections while someone is logged in already using the same IP address and client port. The result is that the above issues do not occur, and anyone connecting will see a "connection timed out" message.
You can change the client port which Neverwinter Nights uses to connect. Make sure that the game is not running, then open your Neverwinter Nights directory (the default on Windows is C:\NeverwinterNights\NWN). Find the file nwnplayer.ini and open it. Note: by default, Windows hides known file extensions, so the file may appear as simply "nwnplayer".
You should find a line which reads something along the lines of:
Change 5120 to some other whole number between 1025 and 65535. It doesn't really matter what number you choose, unless you run a server on your own computer. For example, if you ever intend to host a multiplayer Neverwinter Nights game, you should not use port 5121 (the default multiplayer server port) as your client port.
The idea is that all computers on a network running Neverwinter Nights should have a different client port. So for instance if you have three computers, the first may use port 5211, the second port 5212 and the third port 5213. With such a setup, you will be able to play on Arelith without a problem.
Why isn't this happening to me?
If you are not experiencing this issue and are using the default client port 5120 on multiple computers on your network, it is likely that your router is forwarding outgoing connections via one port to a different port. In this case, even though you may have two connections from your network going through port 5120, by the time it gets to Arelith the server sees the connections coming from two distinct ports, maybe 65005 and 65012. There is nothing to worry about in this case - the port checker on Arelith isn't broken, and you won't cause any of the problems described above.
If you ever use a public network (including, for example, a college campus network) to play Neverwinter Nights online, it's well worth taking precautions and changing your client port setting preemptively. You never know who else might be playing, and you could cause problems for yourself or them if both of you try to play on the same server.
I'm timing out, and nobody else plays on this network!
Very occasionally (the cause hasn't yet been discovered but some people have reported issues) you may find that your connection to the server times out anyway. First, make sure that the server in question is up, and that you can connect to other NWN servers. If you can, then there may have been a problem with releasing your IP/port combination. This will solve itself after a reset, but if you can't wait that long, simply change your client port as described above and you will be able to log in again.