cat /etc/network/interfaces
### Hetzner Online AG - installimage
# Loopback device:
auto lo
iface lo inet loopback

# device: eth0
auto  eth0
iface eth0 inet manual
iface eth0 inet6 manual

auto br0
iface br0 inet static
  address   xx.x.xxx.xxx
  broadcast xx.x.xxx.xxx
  bridge_stp off
  bridge_ports eth0
  netmask   255.255.255.224
  gateway   xx.x.xxx.xxx
  post-up mii-tool -F 100baseTx-FD br0

# default route to access subnet
up route add -net xx.x.xxx.xxx netmask 255.255.255.224 gw xx.x.xxx.xxx br0

iface br0 inet6 static
address xxxx:xxx:xxx:xxx::2
netmask 64
bridge_stp off
bridge_ports eth0
up ip -6 route add xxxx:xxx:xxx:xxx::1 dev br0
down ip -6 route del xxxx:xxx:xxx:xxx::1 dev br0
up ip -6 route add default via xxxx:xxx:xxx:xxx::1 dev br0
down ip -6 route del default via xxxx:xxx:xxx:xxx::1 dev br0