When using multiple systems the indispensable tool is, as we all know, ssh. Using ssh you can login to other (remote) systems and work with them as if you were sitting in front of them. Even if some of your systems exist behind firewalls you can still get to them with ssh, but getting there can end up requiring a number of command line options and the more systems you have the more difficult it gets to remember them. However, you don't have to remember them, at least not more than once: you can just enter them into ssh's config file and be done with it.
For example, let's say that you have two "servers" that you connect to regularly, one at your house that's behind your firewall. Further, let's say that you use dyndns to make your home IP address known, and that you've got ssh listening on port 12022 rather than the default port 22 (and you've got your firewall forwarding that port to the server). So to connect you need to run:

$ ssh -p 12022 example.dyndns.org

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