You can run Citect on Ethernet provided the device in the field is Ethernet and there is a driver available in Citect.
For example, You can run to Omron devices by Ethernet, Controller Link, Sysmac Link, Sysmac Net Link or serial. The drivers are available from Citect free of charge.
If you want to run Allen Bradley you can run Ethernet as well. It will cost you for the drivers as AB charge for them. A licence is required each time.
Many other brands of devices can be run on Ethernet. Check with your Citect rep or at www.citect.com.