I made the server IPv6.

I made the server IPv6.

①Confirm that IPv6 is invalidated.

$ cat /etc/sysctl.conf

# Do not use IPv6
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

②Activate IPv6.
$ sudo sed -i -e "/net.ipv6.conf.default.disable_ipv6/s/^/#/" /etc/sysctl.conf
$ sudo sed -i -e "/net.ipv6.conf.all.disable_ipv6/s/^/#/" /etc/sysctl.conf

③Enable IPv6 on the interface
$ sudo sed -i -e "s/^#//g" /etc/network/interfaces

④Check the config file.
$ cat /etc/network/interfaces
source /etc/network/interfaces.d/*

auto lo
iface lo inet loopback

auto ens3
iface ens3 inet static
address XXX.XXX.XXX.XXX
netmask 255.255.254.0
gateway XXX.XXX.XXX.1
# dns-* options are implemented by the resolvconf package, if installed
dns-nameservers XXX.XXX.XXX.XXX XXX.XXX.XXX.XXX210.188.224.10
dns-search XXX.XXX.XXX.XXX

iface ens3 inet6 static
address 2001:e42:102:1501:XXX:XXX:XXX:XXX
netmask 64
gateway fe80::1
dns-nameservers 2001:e42::2