1. Login to WHM

2. Go to WHM » IP Functions » IPv6 Ranges 

3. Click Add Range 

    3.1 Add Gateway of IPv6 Ranges it like this 2401:8640:c010:700:203:78:xxx:1/128  and checkbox on Reserved  - addresses cannot be assgined

          Then click on Add Ranges   it will get result below.

      3.2 Add IPv6 Ranges , this is add /128 for one IP. Like this 2401:8640:c010:700:203:78:xxx:xxx/128 and check on Available - address can be assigned

            Then click on Add Ranges   it will get result below.