In a network that uses stateless autoconfiguration with router advertisements,
every node will assign to itself a unique IP address. This can be a random
temporary address, or one that is derived from the MAC address of the network
interface.

Such a MAC address uniquely identifies a certain computer. If it's used in your
IPv6 address on a computer that moves between networks, parties you connect to
can use it to track your location even without the use of cookies.

IPv6 Privacy Extensions can be used to randomize the IP address that is used
for outgoing connections. Although your system administrator might not like the
idea of having completely random IP addresses, it's a good way to enhance your
privacy on the internet.


Windows:

    Privacy extensions are enabled by default.

    netsh interface ipv6 set privacy enabled

    http://ipv6int.net/systems/windows_xp-ipv6.html#privacy

Mac OS X:

    sysctl net.inet6.ip6.use_tempaddr=1

    http://ipv6int.net/systems/mac_os_x-ipv6.html#privacy

Linux:

    sysctl net.ipv6.conf.INTERFACE.use_tempaddr=2

    http://ipv6int.net/systems/linux-ipv6.html#privacy

FreeBSD:

    sysctl net.inet6.ip6.use_tempaddr=1
    sysctl net.inet6.ip6.prefer_tempaddr=1

    http://ipv6int.net/systems/freebsd-ipv6.html#privacy