Example code to create an NTP refid from an IPv6 address. RFC5905 describes NTPv4 and how to calculate the refid for an NTP server. As there are only 32 bits available for the NTP refid, the IPv6 address will not fit. Hence the workaround of getting the md5 of the IPv6 address and using the first 4 bytes of the md5 for the refid.
ntp1-pw.heanet.ie has address 193.1.8.106 ntp1-pw.heanet.ie has IPv6 address 2001:770:101:2::2
Reference ID : E8B9A649 (ntp1-pw.heanet.ie)
Stratum : 2
Ref time (UTC) : Sat Oct 01 17:11:42 2022
System time : 0.000000724 seconds fast of NTP time
Last offset : -0.000001201 seconds
RMS offset : 0.000017060 seconds
Frequency : 27.721 ppm slow
Residual freq : +0.000 ppm
Skew : 0.006 ppm
Root delay : 0.000622455 seconds
Root dispersion : 0.000645751 seconds
Update interval : 1040.8 seconds
Leap status : Normal
$ gcc ntp-refid-ipv6.c -l crypto -o ntp-refid-ipv6
$ ./ntp-refid-ipv6 2001:770:101:2::2
First 4 bytes of the MD5 of (2001:770:101:2::2) = ntp refid = e8b9a649