How to use ssh tunnel to reach a machine on a private network?

[Oliver Elphick <olly@lfix.co.uk>]

I wonder if anyone can help me work out how to do this, please:

I have two private networks (192.168.1.0/24) each with a firewall
machine connecting through ADSL to the Internet.  Each private network
can reach the Internet through the firewall (using NAT); therefore no
machine except the firewall is visible from outside (at static IP
addresses allocated by the ISP).

I can, from any machine on either private network, do
"ssh -X remote.firewall.address" and connect to the remote firewall. 
What I am trying to do is to use ssh tunnelling to go direct to one of
the machines on the remote private network, because I need to be able to
run X programs from that machine on my own display.  However, I can't
work out how to do it.

So far, I tried

   ssh -X -L 8877:remote.private.machine:22 remote.firewall.address

(using 8877 as an arbitrary unassigned port) but all that gives me is a
connection to the remote firewall itself.

-- 
Oliver Elphick                                Oliver.Elphick@lfix.co.uk
Isle of Wight, UK                             http://www.lfix.co.uk/oliver
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