FYI: Ipv6 users should use 2.pool.ntp.org as their NTP server
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For those who have ipv6 and want to use pool.ntp.org as the time server for your NTP clients, they should use 2.pool.ntp.org as this is the only one that returns both A and AAAA records for NTP servers. pool.ntp.org is ipv4 only and many people by default use pool.ntp.org
This also works for regional servers. i.e I am in the US so I use 2.us.pool.ntp.org. It also works for the distro specific entries like 2.pfsense.pool.ntp.org and 2.fedora.pool.ntp.org. Only 2 returns v6 entries.