In Vista, if you did not have native IPv6, then certain applications such as Windows Live Messenger demanded IPv6 and so the o/s kicked Teredo into action. I am guessing that an app might insist on sending something to a target IPv6 address, and so a source IPv6 address has to be created somehow for the operation to proceed. It worked well for me. A lot of people moaned about the unreliability of Teredo at some point later on, but I get the feeling that their problems may just reflect problems with UDP-based higher protocols in general, and might not necessarily be confined to Teredo, it’s just that TCP was holding everything together with duct tape, assumptions and retx. If in fact an application is sending way too fast, without TCP then it is maybe no longer getting away with it. Or if there is packet loss in general for no good reason, with UDP alone and lacking extra clue on top of it, then you’re in trouble.