VNC server listens on three ports (5800, 5900 and 6000) what does port 6000 do?

Question

I have a CentOS server with vncserver service configured. netstat tells me that Xvnc listens on 5800, 5900 and 6000 ports.

What does port 6000 do for Xvnc?

Thank you

Solution

That port seems to be used by the X11 graphics system

# grep 6000 /etc/services

x11 6000/tcp X # the X Window System

Maybe xvnc is using some kind of port forwarding for X11