Hi... I've been working and studying computer networks for several years. If my ideas concerned, I have to mention that apart from those categories like Network design, Network security and Network Performance there are also other activities involved like Intelligent Networks which are of utmost importance these days, a combination of artificial intelligence and network engineering or working on the field of producing soft wares to manage networks which needs an in-depth knowledge of software engineering and network engineering. Also there is one another and is working on the subject of network internet-working which means go inside the architecture of OSI/RM or TCP/IP layers and even to produce a new concept.
[edit: renamed subject for clarity.]