Somehow Teredo in Windows Vista and newer overcomes this, but I have not yet found a clear explanation for how. A new, random source port is chosen for the next session.When the communication is complete the source port is no longer in use.Next the source port is randomly changed, with the mapping existing only in the router's memory.First the IP address is changed from the source's non-routable address to the public IP address.Symmetric NAT randomly changes the "outside" source port per session.Teredo doesn't seem to play very nicely with symmetric NAT. By keep tracking of the source port and the IP this allows the Teredo server to reach directly back to the client, essentially bypassing NAT without any configuration required.Teredo acts almost like a dynamic port forwarding service that keeps track of both the public IP address and the source port of the client.Source ports can be prone to change and it is important for the Teredo server to know how to reach back to the client.This interval is randomly varied by the Teredo service.There is a "refresh" interval in which the client verifies that the source port is still valid.This maintains the NAT mapping between the client's source port and the public IP address (the "client's" or router's public IP).
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