auto eth0
iface eth0 inet static
address
netmask 255.255.255.255
gateway 172.31.1.1
pointopoint 172.31.1.1
dns-nameservers 213.133.98.98 213.133.99.99 213.133.100.100

iface eth0 inet6 static
address
netmask 64
gateway fe80::1

reference:
https://docs.hetzner.com/cloud/servers/static-configuration/