VNC Authentication

I try enable Remote Administration, I can connect to SLES using TightVnc Viewer. The problem is : If I connect from internal network LAN, the SLES not asking login authentication just go directly to the dekstop.

But if I connect from Outside Network, SLES always asking to login.

How to set up so SLES always ask to Login even from internal LAN. any article that have a stepbystep explanation ?

Thanks

are you using xinetd to start vnc server?