NCTUns is a high-fidelity network simulator and emulator. By using a novel kernel re-entering simulation methodology, it provides many unique advantages over traditional network simulators and emulators. For this reason, since its public release on November 1, 2002, as of January 20, 2006, more than 4,840 people from 95 countries in the world have registered at its web site (http://NSL.csie.nctu.edu.tw/nctuns.html) to download it. NCTUns is open-source and runs on Linux. It can simulate many different types of networks. It has many high-quality journal and conference papers supporting its simulation methodology, design, and implementation. After six years of development, it has become a high-quality and useful product and has received many academic honors. In this paper, we give a brief introduction to NCTUns. More information about this tool is available at its web site.