Reaching machine from another network (having RDP to another machine in the same network)

view story

http://serverfault.com – Lets say there is Machine A (Ubuntu) in Network A and Machine B in Network B. There is also Machine C (Windows 7) in Network B and I can access it through RDP from Machine A. What I need is to be able to reach Machine B from Machine A (i.e. can ping it, connect to arbitrary port, etc). How can I manage something like this? PS: Network B is a dmz, i.e. I cannot directly access Machine B/C from the Internet, RDP is port-forwarded from dmz frontend server to Machine C. (HowTos)