Referring to many blog posts mentioning Teredo Tunneling Protocol, I know that a teredo server provided by Microsoft, teredo.ipv6.microsoft.com, as a default option in both Windows Vista and Windows 7, is blocked by the GFW (or the routers of China Unicom network, I don't know).
To avoid the blocking, you could set your teredo client server as following:
Starting Command Prompt as administrator, entering
teredo.remlab.net is a teredo server located at France, it is available now and a best choice at China Mainland. Other options available here.
netsh int teredo set state server=teredo.remlab.net.
If you messed up, you could always use the following command to reset all the settings.
Reset teredo settings:A bunch of IPv6 addresses of Google services is listed here. The information will be updated irregularly. You may cannot access this address owing to the blocking, try it using a proxy.
netsh int teredo set state default server=default
Reset IPv6 settings:
netsh int ipv6 reset
Happy tunneling! 祝翻墙愉快！