This chapter presented a high-level overview of modern operating system concepts and the viability of UNIX-like operating systems, just enough to put Cisco IOS Software into perspective. Concepts such as forwarding, routing, switching, and signaling were introduced. Aspects such as kernel architectures, kernel-space and user-space operation, and real-time and embedded operating systems were discussed as well. In addition, monolithic, micro-kernel, and modular kernel features were discussed, and GNU packages and distributions were put into perspective.

Finally, the dominating open-source UNIX-like operating systems?OpenBSD, FreeBSD, NetBSD, and Linux?were discussed and compared to some specific aspects of Cisco IOS Software.