Dec 01 01:33 2016, Michiel van der Vlist wrote to Nicholas Boel:
MvdV> The following comes to mind. I have been assigned a /56. So I can MvdV> have 256 Iv6 subnets. But... I have not as yet discovered a way to MvdV> use anything but the first /64 of that assigment with my MvdV> modem/router. How about you?
PD ;) Try a DHCP client supporting prefix delegation and ask the cable modem/router for a prefix.