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.