KVM | debian | building a bridge

You did set up a kvm and have one physical interface with IP 192.168.100.121, i.e. eno1?

To get a bridge tune your /etc/network/interfaces like this:

auto lo
iface lo inet loopback

iface eno1 inet manual
auto br0
iface br0 inet static
  address 192.168.100.121
  netmask 255.255.0.0
  gateway 192.168.100.1
  dns-nameservers 192.168.100.2 8.8.8.8
  dns-search inbbradio.de
  bridge_ports eno1
  bridge_stp off
  bridge_fd 0
  bridge_maxwait 0


# END

That means:

Yout establish a virtual bridge br0 with ip 192.168.100.121 and your physical interface eno1 is docked on the inner side of this bridge.

YOURLAN << br0WITH192.168.100.121<<>>YOURPHYSICALeno1