When you first download a PLC program of a STEP 7 Basic project to a network which contains several S7-1200 PLCs a situation may occure in which more than one PLC shares the same IP address.
This means you will miss one or more PLCs. In this case delete the IP addresses of all PLCs to be able to access them by their MAC addresses.
Delete IP address
Delete the IP address of a S7-1200 PLC using a SIMATIC MC memory card.
Please find a detailed description of the procedure in entry ID 40588092.
Identify by MAC
To address a PLC by its MAC address enable the checkbox show all accessible devices in your dialog box Extended download to device and select a MAC address from the list.
Download to PLC
Select a MAC address from the list of accessible devices and click the load button.
A factory-new PLC also has no preset IP address.