Using remote desktop on CentOS5(RHEL5) looks simple until you face
'unable to connect to host: No route to host (113)'
This is caused by firewall setting of target machine, which is normally a machine you want to connect to.
It's very simple. Login to the target machine and click the menu 'System/Administration/Security level and Firewall'
and click 'Other ports' then add 5900 port.
Once you have done so far, try again.