Folks, when the peer is IPv6, methinks that the reference ID is a 32 bit hash.
Update:
Since NTP was designed when IPv4 was the only game in town, using the IP address to detect time loops was a really great idea. For S2+ packets, we displayed the
refidas an IPv4 address, which meant it was obvious where each machine was getting its time from. But then came IPv6, and a way needed to be found to translate the 128 bits that make up an IPv6 address into a 32-bit field. The solution that was decided on was to use the first 4 bytes (32 bits) of the MD5 hash of the IPv6 address.
(Source: NTP's REFID)