Go to the “Networking”→"Networks" section and click on the “Create new network” button:
Specify a unique network name, description, subnet (in CIDR format).
At the next step if you want to get gateway IP automatically check “Automatically gateway IP?”, if not, specify it manually and enter DNS addresses.
By default first IP is reserved as a default gateway, and second one reserved as a DHCP agent.
You can change the name or description by clicking “Edit” from the “Edit” column of the appropriated subnet.
If you want to edit the subnet itself, press on the appropriated subnet.
Under the “Networking”→"Networks" section create or choose a subnet.
From the right side click on the “Attach a port” and select VM name.
Or under the “Virtual Machines”→"Networks" section click on the “Attach network”