The IPV6 protocol has a feature called "Extended Unique Identifier" or EUI-64 witch in short uses the MAC address of the network card when choosing an IPV6 Adress.
Proof: http://www.youtube.com/watch?v=30CnqRK0GHE&NR=1 at 7:36 video time.
If you want to be anonymous on the internet (so that nobody can find you when you download something, etc.) you need this EUI-64 to be bipassed in order for the MAC address not to be discovered by harmful third parties on the internet and for privacy.
How do you avoid EUI-64 MAC address usage in IPV6 selection in Ubuntu? Also for DHCP IPV6?