The Virtual Network System it is a platform to facilitate teaching of network infrastructure. Unlike existing simulators, the system operates with real IP packets in real time and supports integration with the Internet. It allows students to build real routers in user space (using C,C++,Java, Click or even in hardware) that route real Internet traffic. It is currently in use at multiple classes at Stanford and at other Universities. This talk will describe the capabilities of VNS and explore how it may be used to build networks of XORP or Click routers. The VNS is a joint work with Guido Appenzeller, Dan Wendlandt and Nick McKeown.