Enabling Raspberry Pi IPv6 Privacy Extensions in Raspbian Jessie
If you have switched on IPv6, then the interface identifier of the IPv6 addresses is formed from the MAC address (hardware address). Unfortunately, it is always the same, which is why the respective clients or hosts can be identified by this address. For privacy reasons, that’s a problem.

That’s why with Privacy Extensions for IPv6, there’s an extension that protects privacy. Under Raspbian Jessie, IPv6 is on by default and Privacy Extensions is enabled.

• Privacy Extensions (IPv6)