Why do some Teredo ports not work?

i just need to get reply from pinging google's ipv6 server with this command: ping -6 ipv6.google.com sometimes this does work and sometimes it doesn't and i've found out that it depends on the port number Teredo is using. windows will usually give you a port around the ~50,000 range, and sometimes the port it allocates for you might not workcso you have to go into the terminal and...