Nikon D6 Reference Manual page 646

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Firewall Settings
TCP port 15740 and UDP port 5353 are used for connections to
computers. File transfer may be blocked if the server firewall is not
configured to allow access to these ports.
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Voice Memos
Voice memos will be included when the associated pictures are
transmitted. They cannot, however, be uploaded separately.
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Interval Timer Photography
Choose a long standby time before starting the interval timer, as upload
will be interrupted if the standby timer expires while interval-timer
photography is in progress.
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Turning the Camera Off
The camera saves transfer marking when turned off and resumes upload
when next turned on.
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Upload to Computers and FTP Servers