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.

Also you should understand what the Domain Name System (DNS) is, what it provides and how to use it.

You should at least understand how to use tcpdump and what it can show you. Otherwise, network debugging will very difficult for you.