We describe a sequence of five experiments on network security that cast students successively in the roles of computer user, programmer, and system administrator. Unlike experiments described in several previous papers, these experiments avoid placing students in the role of attacker. Each experiment starts with an in-class demonstration of an attack by the instructor. Students then learn how to use open-source defense tools appropriate for the role they are playing and the attack at hand. Threats covered include eavesdropping, dictionary, man-in-the-middle, port scanning, and fingerprinting attacks. Defense skills gained by students include how to forward ports with OpenSSH, how to prevent weak passwords with CrackLib, how to salt passwords, how to set up a simple certifying authority, issue and verify certificates, and guarantee communication confidentiality and integrity using OpenSSL, and how to set up firewalls and IPsec-based virtual private networks. At two separate offerings, tests taken before and after each experiment showed that each has a statistically significant and large effect on students' learning. Moreover, surveys show that students finish the sequence of experiments with high interest in further studies and work in the area of security. These results suggest that the experiments are well-suited for introductory security or networking courses.