Can somebody give me an overview of what network testing is exactly? What is actually tested in a network?
To get started, ask yourself these questions:
"What is a network?"
"How does a network work?"
"What are the physical components of a network?"
"What are the software components of a network?"
"What is the purpose of the network?"
"To what does the network connect?"
"Whose tasks are being served by the network?"
"What is the nature of those tasks?"
"What are the -ilities -- capability, reliability, usability, scalability, maintainability, and so on--that would be valuable to the customer (not just the users) of network like this one at any price?"
"What are the -ilities of a network like this one, at a price like this one?"
That's a start; there are other questions you can ask too. Then ask yourself "How would we find out if our network matched the answers to the questions above to the degree required by the people who are asking us to do the work?". Then you've got something like a test plan. Testing anything is about posing questions about the thing and answering them.