Hetzner has a very unique way of assigning additional IP ranges to their dedicated server, which, in order to use these IPs for a virtual machine server, requires a virtual bridge.

Take a look at ‘/etc/sysctl.conf’ and make sure that the following two lines aren’t commented out:

net.ipv4.ip_forward=1
net.ipv6.conf.all.forwarding=1

auto lo
iface lo inet loopback
iface lo inet6 loopback

# Main physical NIC
auto eth2
iface eth2 inet static
	address 95.216.45.147
	netmask 255.255.255.192
	gateway 95.216.45.129
	up route add -net 95.216.104.224/28 netmask 255.255.255.240 gw 95.216.45.147 eth2
# Bridge NIC for VMs 
# Source: https://blog.no-panic.at/2016/08/09/proxmox-on-debian-at-hetzner-with-multiple-ip-addresses/
auto virbr1
iface virbr1 inet static
	address 95.216.45.147
	netmask 255.255.255.255
	bridge_ports none
	bridge_stp off
	bridge_fd 0
	bridge_maxwait 0
	pre-up brctl addbr virbr1
	up ip route add 95.216.104.224/28 dev virbr1
	up ip route add 95.216.104.225/28 dev virbr1
	<add new route per IP being used>

auto ens3
iface ens3 inet static
	address <IP from /28 subnet>
	netmask 255.255.255.240
	dns-nameservers 1.1.1.1
	post-up ip route add 95.216.45.147 dev ens3
	post-up ip route add default via 95.216.45.147 dev ens3
	post-down ip route del default via 95.216.45.147 dev ens3
	post-down ip route del 95.216.45.147 dev ens3

Add to KVM config via “$ virsh edit <VM name>” place in the devices section

<interface type='bridge'>
      <mac address='<generate this with "$ hexdump -vn3 -e '/3 "52:54:00"' -e '/1 ":%02x"' -e '"\n"' /dev/urandom">'/>
      <source bridge='virbr1'/>
      <model type='rtl8139'/>   # virtio works here too
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
</interface>