Due to the shortcomings of the budget, laboratory resources, and instructors, current computer network courses can not fulfill both the requirements of the computer industry and the research community. Even though computer network is one of the rapidly changing areas, its curriculum could not keep abreast with this change and thus is limited to teach students the theoretical networking concepts. In this paper, we propose Kaist Educational Networking System (KENS), which is an instructional network simulator consisting of a minimal set of operating system components, socket API, the transport layer, network layer and datalink layer. Reading configuration files, KENS generates a group of virtual nodes forming a virtual network. Students are challenged to implement the transport and network layer, as well as the application program running on top of the KENS kernel. By testing and debugging the programs in the virtual network, they can easily see how the different layers in the network stack are interacting each other and what kind of challenges are bringing up. Since KENS is simple and running as a user level process, students can easily delve into learning and implementing various layers without the burden of hacking OS kernel and KENS itself. In addition, KENS allows a lot of students to share one laboratory machine, so that the department does not have to spend their budget to buy expensive simulators or communication hardware. We believe that KENS is an excellent low-cost alternative method supporting the broad context of networking-related courses.