I am a 4th year Network Security and Systems Administration major and I have found the following to be pretty useful:

Cisco's Packet Tracer is awesome for simulating and virtualizing network connections and network devices. It also helps one get pretty familiar with Cisco's IOS. http://www.cisco.com/web/learning/ne...ketTracer.html

I also have found VMware Workstation with the Virtual Network configuration options on top of Virtual Teams (logical groups of VMs) to be pretty useful with setting up virtualized operating systems with different network abilities. Virtual Network Adapters can be created in almost limitless quantities for each Virtual Machine, and custom configurations (bridged,nat,host only,...) can be configured for an adapter connected on Virtual LAN Segments. All of this makes it pretty easy to build a real network in a virtualized environment. Another tool which is pretty useful in a VMWare network is the FreeSCO (single floppy router) which can easily be installed as a Virtual Machine. In many cases I use this VM to route between two subnets or lan segments and it normally has an additional NAT'd adapter which I configure as the router's default gateway. This in turn allows for the entire virtual network to get access to the network (internet) that your host machine is connected to, but confines it to a single entry point on this router. http://www.freesco.org/

I hope this all makes sense, I'm not the best about getting my ideas and suggestions out of my head and into forum posts.