in a unix network you can log on to other machines across a network and switch users but you can only have full super user (root) permissions on the machine you are physically on. you cant be super user on a remote machine.
Really ? Who told you that utter load of bull ?
I occasionally have reason to sit in a warm quiet room in front of the television and manage servers located in noisy server rooms many miles away.
I can login to the servers remotely, I can escalate to full root privileges in a matter of a few keystrokes. All using the OS's built-in tools.....
$ sudo su -
#whoami
root
#
Outside of the OS's built-in tools, I can also rebuild a server from scratch remotely, but that's another story.
If I were to be generous and give you the benefit of the doubt, perhaps you're confusing yourself with "single user mode", i.e. where the network is disconnected ? (Although there are ways around that as per my second paragraph)