When I was teaching computing to middle school and high school students we didn't do much on networking. Partly because there was no real curriculum on it and partly that they had already set up LAN parties and they now knew almost as much as I did.
Times have changed and "the network" is the most important thing in a school, college, university or workplace (watch what happens when it goes does down for a day). So teaching the network is now an important part of the syllabus for computer studies.
So here are a few network Visualisation tools.
Warriors of the Net - a cool animation / visualisation tool of what a Packet switch network looks like from a packet point of view.
CNET - From my home town of Perth - cnet enables development of and experimentation with a variety of data-link layer, network layer, and transport layer networking protocols in networks consisting of any combination of wide-area-networking (WAN), local-area-networking (LAN), or wireless-local-area-networking (WLAN) links.Available for linux and Apple OS only.
INET framework -
GINI - GINI (GINI is not Internet) is a toolkit for creating virtual micro Internets for teaching and learning computer networks.
CLACK - Clack is a virtual network environment for visualizing network behavior and router internals.