Linux is a Unix-based computer OS and was originally designed as free software for open-source development. Its source code can be freely modified, used and redistributed by anyone under the GNU Public License. Several GUIs run on top of Linux, including K Desktop Environment and GNU Network Object Model Environment. Of the many distributions of Linux, the most-popular enterprise versions include those from Red Hat (Red Hat Enterprise Linux), Novell (SUSE Enterprise Linux), Ubuntu and Debian, and regional ones, such as Mandriva, Red Flag and Asianux. There are two Linux subsegments, Linux (client) and Linux (server), in our segmentation.