Under Network→Networks click the "Create Network" button
Fill out the "Network Name" field with the name of the network.
Click Next or click on "Subnet" Tab.
Fill out the "Subnet Name" field with the name of the subnet.
Fill out the "Network Address" field with netork address in the CIDR format, for example 192.168.0.0/24.
Cick "Next" to "Subnet Details" tab, where You are able to add custom configuration of DHCP allocation pools, DNS servers and static routing. you can leave it as default.