connecting to vnc-server outside LAN

[Ronald Saa]

Hello everyone,
I can connect to (by vnc) any computer within the LAN, 
just enter the IPaddr at the Viewer prompt.

But when I try to connect (by vnc) to anyone outside the LAN,
I get the "Attempting to Connect" window...
Then the "Unable to connect to host: Connection timed out (10060)"
error

What is the most common reason for this error?
It's driving me nuts.

Thank you in advance for any assistance.
Noni