Hello Cecil, sales2161
Sales has got it right. The prefix just locks in the first 64 bits of the address, or the “network portion” of the address if we want to use IPv4 terminology. The rest can be random, or EUI-64 or even be assigned statically by a DHCPv6 server. It can be any of these because IPv6 also employs the Duplicate Address Detection (DAD)mechanisms, a part of the Neighbor Discovery (ND) Protocol used in IPv6, to ensure that no duplicates have been assigned. In this sense, with IPv6 you don’t have to keep track and manage addresses as much, the IPv6 mechanisms here take care of that.
More about DAD and ND can be found at this lesson:
I hope this as well as sales2161’s posts have been helpful!
Laz