As society becomes increasingly digitalized, many high-school curricula now cover the mechanisms of computer networks, including packet retransmission and routing. University courses can allot enough time for CS majors to master professional network simulators and experience hands-on practice. High school classes, by contrast, cannot devote such extensive time to each topic, making general-purpose simulators impractical. To address this limitation, we developed ProtoSim, an educational network simulator tailored to novice learners. ProtoSim visualizes data flow with intuitive animations and provides mission-based exercises with automatic feedback, enabling students to explore network mechanisms autonomously and effectively within limited class time. This paper specifically focuses on visualizing packet retransmission and routing that prior version of ProtoSim did not support. We conducted an educational practice with 59 high school students. The results of the pre- and post-tests showed that students were able to describe the aforementioned topics in their own words after using ProtoSim. Furthermore, based on students’ subjective evaluation and observations during the exercises, it was found that ProtoSim’s problem-based format effectively encouraged students to engage in trial and error and to think independently, serving as a catalyst for deeper understanding of network mechanisms.