DMX was designed to control up to 512 channels (a universe) of lighting values over a single cable. It worked well for many years but eventually outgrew its 512 channel limit, and lighting desks supporting several DMX universes began to appear. Soon, even this was not enough as the development of channel hungry fixtures progressed and designers needed more channels than DMX could offer.
Art-Net was created by Artistic Licence to overcome the channel restriction of DMX while still utilizing its structure. It allows multiple DMX universes to be transported over a single Cat5 cable using ethernet technology.
In the early days, DMX signals from a lighting desk were converted to Art-Net using a separate interface product. The signals were then transported over an ethernet network and converted back to DMX before entering each fixture. However, manufacturers quickly saw the benefits of Art-Net and started to support the protocol in their new lighting controllers. As a result, native support for Art-Net in consoles, dimmers and moving lights is now quite common, so increasingly the DMX512 universe is a virtual concept.
For more information about the protocol, please visit http://art-net.org.uk