VNCVIEWER Error Message: vncviewer unable to open display

I try to connect Remote system(GUI- Remote Desktop) through ssh for that Purpose I used VNC.
VNC - Virtual Network Computing,Remotely control another Computer Like Remote Desktop.
It is used to Trouble shooting the problem.While I try
[root@remote ~]# ssh 151.8.19.140
Last login: Tue Feb 1 15:47:03 2011 from 151.8.19.145
[root@ha2 ~]# vncserver
New 'ha2.hcl.in:5 (root)' desktop is ha2.hcl.in:5
Starting applications specified in /root/.vnc/xstartup
Log file is /root/.vnc/ha2.hcl.in:5.log
[root@ha2 ~]# vncviewer :5
VNC Viewer Free Edition 4.1.2 for X - built Jan 26 2009 11:52:08
Copyright (C) 2002-2005 RealVNC Ltd.
See http://www.realvnc.com for information on VNC.
vncviewer: unable to open display "

ERROR MESSAGE

vncviewer: unable to open display

PROBLEM

while Login through ssh we Didn't enable the X11.
X11 refers to the GUI - Graphical User Interface
SOLUTION
while login though ssh use the option '-X' To enable X11.
-X enable X11
-x Disable X11.
Then it will work
[root@remote ~]# ssh -X 151.8.19.141
Last login: Tue Feb 1 10:18:55 2011 from 151.8.18.160
[root@ha1 ~]# vncserver
New 'ha1.hcl.in:4 (root)' desktop is ha1.hcl.in:4
Starting applications specified in /root/.vnc/xstartup
Log file is /root/.vnc/ha1.hcl.in:4.log
[root@ha1 ~]# vncviewer :4
VNC Viewer Free Edition 4.1.2 for X - built Jan 26 2009 11:52:08
Copyright (C) 2002-2005 RealVNC Ltd.
See http://www.realvnc.com for information on VNC.
Tue Feb 1 10:19:26 2011
CConn: connected to host localhost port 5904
CConnection: Server supports RFB protocol version 3.8
CConnection: Using RFB protocol version 3.8
Then I got the Graphical Window For the Remote System.