In order to activate basic IPv6 functionalities, you first have to enable it (disabled by default).

mattieu@IRIS: ~ $ sudo systune ip6_enable 1
        ip6_enable = 0 (0x0)
        Do you really want to change ip6_enable to 1 (0x1)? (y/n)  y
 
In order for the change in parameter ip6_enable to become effective,
reboot the system

Then you have to reboot the system as it was asked.