X Window System is a system for implementing a window-based user interface on bit-mapped displays. X11, the most current and widely used version of X, is designed to communicate all the information necessary to operate a window system over a single asynchronous bi-directional stream of 8-bit bytes. The X protocol specifies four types of messages but named extensions can also be defined to extend the system. X11 usually runs on TCP port 6000, 6001, 6002, etc.