(1)When all related IPv6 addresses are removed,
   then remove the associated IPv6 prefix.
(2)When multiple IPv6 link local addrs exist for a same
   interface , then let its IPv6 prefix have multiple
   interface id, and create multiple IPv6 global addrs with same
   interface id.
(3)When a new IPv6 link local addr is assigned for an
   interface, then let its IPv6 prefix also have the
   interface id of the new IPv6 link local addr, and
   create new IPv6 global addrs with same interface id.