Controlling A Machine Inside A Firewall From Outside...

[Rick Strong]

Greetings and apologies if this this subject has already been rehashed 
too many times...

I need to be able to control a Windows machine living inside of a 
firewall. I can (attempt to) contact it either from within another 
firewall or from a machine with a real IP address.  I have already done 
the reverse: set up a server inside of the firewall, start a listener 
outside, and on the server added the machine outside as a client, at 
which point I can see the machine outside from inside.

Is there a way to reverse this, and see/control the machine inside the 
firewall from outside?

Thanks for any info and for your patience...

Rick Strong