@phireph0x 

This is actually not somthing that is possible.  Essentially this is because every IPv6 address on a LAN is already a reserved address.  What I mean by this is the last 4 octets of an IP V6 is the interface's MAC ID.  Thus making the IPV6 address unique to that device.  

A much more detailed explanation for this can be found in this article by Oracle. 

According to this article, reservations are supported via a DHCPv6 Server in the Stateful configuration

https://www.ictshore.com/free-ccna-course/dhcpv6-basics/

@phireph0x 

Its been a while for my Cisco CCNA skills but if I'm reading that correct DHCPv6 Stateful is just refering to an IPv6 address being assigned by the DHCP server,  which is still dynamic addressing, I looked further into our data and at the very least our home products do not have the abilty to do address reservation with IPv6, our SMB line might but we don't deal with those product so i would not be able to say for certain. 

@Carl  Please add IPv6 reservation as a feature request.  I'm running an internal DNS server with both IPv4 and IPv6 addresses.  I have the IPv4 address reserved via DHCP from my Deco M5, but don't have the ability to add the IPv6 reservation for the same server.  Its IPv6 IP has changed a few times, which causes issues with DNS requests from IPv6 devices on my network.

@phireph0x 

Do you know what part of the address changed?  the Prefix Delegation, or client end?  

@Carl the client end.  My IPv6 addresses are changing for all clients.  How can I ensure that these are maintained?

@phireph0x 

As this was a concern in March and there have been a few Firmware updates is this still an concern that needs to be addressed?

@Carl Yes, this issue still persists.  I've updated to the latest firmware as well.

@phireph0x 

I've reached out to our engieeers to have them help address this then.  I will let you know when i hear back from them.

@phireph0x 

This is the response our system engineering team.  Please let me know if this address your question.  

"In a normal case, the IPv6 server will assign the prefix to the client, and then the client will create its own IPv6 address based on the prefix. However, the prefix from the ISP/front-end router is not fixed, it will be updated regularly, if you have configured address reservation for the current IPv6 address, once the prefix is updated, the old IPv6 address will be useless, in this case, there is no sense to reserve an IPv6 address for the client."