How to connect to server on strange port, or when you're behind a firewall

situation

cpuxxxx -> FW -> intermediateserver -> targetserver

22......-> 22 -> 22.................-> portnumber_on_targetserver

On cpuxxxx:

ssh -L portnumber_on_targetserver:targetserver:portnumber_on_targetserver username_on_intermediateserver@intermediateserver

Leave terminal open!

Then on cpuxxxx again:

ssh -YC username_on_targetserver@localhost -p portnumber_on_targetserver

That's it!!