Proxmox 4.x /etc/network/interfaces

interfaces

### Hetzner Online GmbH - installimage
# Loopback device:
auto lo
iface lo inet loopback
iface lo inet6 loopback

# device: eth0
auto  eth0
iface eth0 inet static
  address   148.251.126.177
  netmask   255.255.255.224
  pointopoint 148.251.126.161
  gateway   148.251.126.161

iface eth0 inet6 static
  address 2a01:4f8:210:21b0::2
  netmask 64
  gateway fe80::1
  up sysctl -p

# Bridge vmbr0
auto vmbr0
iface vmbr0 inet static
  address   148.251.126.177
  netmask   255.255.255.255
  bridge_ports none
  bridge_stp off
  bridge_fd 0
  # default route to access subnet
  up route add -net 148.251.156.128 netmask 255.255.255.248 gw 148.251.126.177 vmbr0

iface vmbr0 inet6 static
  address   2a01:4f8:210:21b0::2
  netmask   64