Requirements (2)

Where to assign addresses
/64 to ppp link         /64 to ether            /48 to behind

ISP router              ISP router              ISP router
  |La, Ga                 |La                     |La   
  | ppp link /64          | ppp link              | ppp link
  |Lb, Gb                 |Lb                     |Lb
Cust computer           Cust router             Cust router
                          |Gb                     |Gb
                        ==+=== Gx/64            ((()))) Gx/48
/64 to ppp link
Not recommended, advocates IPv6-to-IPv6 NAT
/64 to ether behind the router
/64 to (virtual) ether behind the router
Restricts customer evolution
/48 to ether behind the router
Our recommendation
ISP does not care about the customer topology.