Xlib: No protocol specified

It can be frustrating when you try to start xwindow application using Xming XServer on Windows XP/7/Vista but encountering connection refused error. Here is an example of the error:

[oracle@oel5 /]$ dbca
Xlib: connection to "192.168.0.100:0.0" refused by server
Xlib: No protocol specified

Here is a quick tip to fix this issue:
In Windows, look for the shortcut to start Xming. Right click your mouse to go to properties.
add -ac to your XMing shortcut:

"C:\Program Files (x86)\Xming\Xming.exe" :0 -clipboard -multiwindow -ac

The -ac parameter allows everyone to connect.
restart Xming and try again.