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.