Step 1: After logging in to the admin interface, go to Interfaces and Routing -> IPv6, and enable “IPv6 status”. As my UTM recieve a IPv6 prefix from my ISP, i end up with this: Step 2: Assign an IPv6 address to your internal interface. This address should be the first IP in your assigned IPv6 prefix. To calculate your range, copy your delegated prefix and go to this calculator. Notice that i have used a /64-prefix internally. Step 3: Go to Interfaces and Routing -> IPv6 -> Prefix Advertisement. Select your internal interface. Then add the IPv6 address of your DNS Server, or the IP of your internal interface of your UTM handes DNS itself. And voila, your internal clients should now be able to recieve IPv6 addresses. I recommend rebooting both the UTM and any clients if they do not get any addresses at this point.