Virtual Machine Manager (virt-manager) doesn’t automatically start your virtual networks. This leads to the following error when starting a vitual machine
Error starting domain: Requested operation is not valid: network 'default' is not active
To solve this error, on Virtual Machine Manger go to Edit->Connection Details->Virtual Networks, select the required network (‘default’ in our case) and press the Start Network button (has a play-button icon). You can avoid having to go through this process by ticking the Autostart checkbox, which will make the network start automatically at boot.