4.2. Prefix assignment options
To keep addressing plans usable and understandable, and to align with DNS reverse zone delegations, the size of the delegated prefix should align with a nibble boundary. Each hexadecimal character in an IPv6 prefix represents one nibble, which is 4 bits. The length of a delegated prefix should therefore always be a multiple of 4.
A single network at a customer site will be a /64. At present, RIR policies permit assignment of a /48 per site, so the possible options when choosing a prefix size to delegate are /48, /52, /56, /60 and /64. However, /64 is not sustainable, it doesn't allow customer subnetting, and it doesn't follow IETF recommendations of �at least� multiple /64s per customer. Moreover, future work within the IETF and recommendations from RFC 7934 (section 6) allow the assignment of a /64 to a single interface (https://tools.ietf.org/html/draft-ietf-v6ops-unique-ipv6-prefix-per-host-07).
from
https://www.ripe.net/publications/docs/ripe-690#4-2-...
Comms is hard 