It redirects incoming connections on port 4689 to port 3689. This allows for the iTunes music sharing server normally run on port 3689 (but with limited access), to be reached on port 4689 globally. The local network limitation in the iTunes sharing server is achieved by sending server responses with a low IP time-to-live. By doing a local port redirection, the server responses are sent with the normal TTL, and the "subnet" limitation is eliminated.
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