By default, KVM provides a NAT network for VMs. This documents the setup of a bridge interface, to allow VMs on the host to join the network of the host, instead of being behind a NAT (which is default).
In 18.04 this is accomplished by using the netplan network configuration tool, which was not used in the previous LTS.
Most installations will have a
50-cloud-init.yaml by default, but you may have to
adjust if not.
The bridge interface will replace the main interface, by default.
sudo apt update && sudo apt install -y bridge-utils
Open the file:
sudo nano /etc/netplan/50-cloud-init.yaml
By default, it will look something like:
network: ethernets: enp1s0: addresses:  dhcp4: true enp2s0: addresses:  dhcp4: true optional: true version: 2
First, disable DHCP on the interface you wish to bridge. Set
dhcp4: true to
Add this to the bottom, noting the interface which it should be bridged to. To bridge with a static IP: (NOTE: UNTESTED)
bridges: br0: interfaces: [enp0s3] dhcp4: no optional: true addresses: [188.8.131.52/24] gateway4: 184.108.40.206 nameservers: addresses: [220.127.116.11]
Or, to bridge with DHCP:
bridges: br0: interfaces: [enp1s0] dhcp4: true optional: true
The final file will look like:
network: ethernets: enp1s0: addresses:  dhcp4: no enp2s0: addresses:  dhcp4: true optional: true version: 2 bridges: br0: interfaces: [enp1s0] dhcp4: true optional: true
To apply the configuration:
sudo netplan apply
Or to apply with debugging:
sudo netplan --debug apply
This will disable the nic you mention, and enable the new bridge nic. This will change the new mac address, and the IP address, so you will have to update the DHCP reservation if this is being used.
Next, confirm the bridge is set up properly:
sudo networkctl status -a
br0 should be in state:
Similarly, check the ip addresses:
br0 should have an ip address associated.
Finally, to connect a VM to the bridge network, when creating the VM in
select “Specify shared device name”, and enter
br0 in the text field.