assign / implement ip6 OVH Ubuntu

ubuntu (16|18).04 - network interface (ifup down)

# /etc/network/interfaces.d/50-cloud-init.cfg
# command : ifup -a
# 2001:4860:4860::8888 2001:4860:4860::8844 is a google dns

iface ens3 inet6 static
	address ipv_6
	netmask 64
	gateway ipv_6_gateway
	dns-nameservers 2001:4860:4860::8888 2001:4860:4860::8844

post-up /sbin/ip -6 addres add ipv_6/64 dev ens3
post-up /sbin/ip -f inet6 route add ipv_6_gateway dev ens3
post-up /sbin/ip -f inet6 route add default via ipv_6_gateway

pre-down /sbin/ip -6 addres del ipv_6/64 dev ens3
pre-down /sbin/ip -f inet6 route del ipv_6_gateway dev ens3
pre-down /sbin/ip -f inet6 route del default via ipv_6_gateway

ubuntu >= 20 (netplan)

#/etc/netplan/50-cloud-init.yaml
# test
# command: netplan try
# apply: netplan apply

network:
    ethernets:
        ens3:
            dhcp4: true
            dhcp6: false
            match:
                macaddress: mac_address # just ignore or do not change this
            set-name: ens3
            addresses:
                - ipv_6/64
            gateway6: ipv_6_gateway
    version: 2