Board Thread:Help Desk/@comment-5833416-20121208162651/@comment-4674838-20121208194616

I'm not sure what exactly on your computer is blocking it, so you may have to try some different things.

One idea is that you can check your Windows Firewall to make sure you've allowed Terraria as an exception. You can find that in Start -> Control Panel -> System and Security section (or something similar, that's how it is for me on Windows 7, if you have different operating system might be slightly different, but look around in the control panel)

Also if you have any anti-virus software on your computer like Norton or McAfee or Avast or something, you might want to check your anti-virus settings to make sure it doesn't have a firewall or something that might be blocking Terraria.