You should be familiar with the major Unix tools e.g. grep, awk, find, ... , and know about their most commonly used command-line options.
You should know about layers, protocols, addresses, cables, plugs, etc. If you are new to this field, here is one good starting point for you: http://www.rigacci.org/docs/biblio/online/intro_to_networking/book1.htm
You should definitely have some experience in IPv4 configuration, otherwise it will be hard for you to understand what is really going on.