By default, Raspian comes unconfigured for IPv6. What a shame. To enable IPv6 support just follow these steps:

Enable the IPv6 kernel module by typing this in your shell:

sudo modprobe ipv6

Now, enable IPv6 by default after boot:

sudo echo "ipv6" >> /etc/modules

If needed, adjust your network configuration file by adding IPv6 here. Options are auto or dhcp. In my case, the modem acts as an IPv6 router (XS4ALL fiber). So adding this to the network options enabled DHCP:

sudo echo "iface eth0 inet6 dhcp" >> /etc/network/interfaces

Now reboot with:

sudo shutdown -rf now