temp add ip

sudo ip addr add x.y.z.w dev eth0

add ip permenently to interafce

Note: In the /etc/network/interfaces.d/, the first files in alphabetic order have more priority. Usually, the default interface file there will start with 50-. So you can use 40 for more priority config and 60 for low priority ones.

Hetzner Suggests

Docs

touch /etc/network/interfaces.d/60-my-floating-ip.cfg
nano /etc/network/interfaces.d/60-my-floating-ip.cfg

auto eth0:1
iface eth0:1 inet static
    address your.Float.ing.IP
    netmask 32

sudo service networking restart

New method

You can only use the network interface twice instead of adding :1 to it like this:

iface eth0 inet static
    address YOUR_IP_ADDRESS/THE_SUBNET_MASK

check the interface is virtual or not

If it returns a true physical mac address, it is physical (not virtual)

ethtool -P eth0

list of virtula devices

ls -a /sys/devices/virtual/net