This paper presents Netsim, a web-based game intended to teach high school aged children the basics of network routing and how common attacks are performed against it by hackers. Netsim is implemented in the form of a network simulator, with levels depicting how common protocols operate, and accompanying tutorial text ex- plaining the protocol or level. Users craft network pack- ets, with a focus on manipulating the header fields, and inject them into the network via computers they control. Goals of the game include spoofing a source address to steal data, or inducing a smurf attack to perform a dis- tributed denial of service. We present a technical description of the game and how it is implemented. We provide a case study of our experiences running the game as a workshop for both high schoolers and educators several times, and the im- provements we made to Netsim as a result. Netsim is available free and open source, and is also available as a hosted webapp that is free for users to access.