Thank you, @Harold Ritter I am aware that DHCPv6 has been supported for a long time and that the version on my ISR4321 is old, but I do not understand why my particular version of the IOS does not recognize the commands I mentioned in my previous pos...

Thank you @Sivagami Narayanan I know I am coming in late to this, and I should really update my IOS, BUT, I have IOS 15.5 on ISR4321 routers, and the address prefix command is an unrecognized command.  In your example, it seems you are using IOS 15.0...