The audience for this book is intended to be the average network or system administrator whose job duties are not specifically security and who has at least several years of experience. This is not to say that security gurus won't get anything out of this book; there might be areas or tools discussed that are new to you. And likewise, someone just getting into IT will learn quite a bit by installing and using these tools. The concepts discussed and techniques used assume a minimal level of computer and network proficiency.
There is also a broad group of readers that is often overlooked by the many open source books. These are the Windows system administrators. The info-security elite often has a certain disdain for Windows-only administrators, and little has been written on quality open source software for Windows. However, the fact remains that Windows servers make up the lion's share of the Internet infrastructure, and ignoring this is doing a disservice to them and the security community at large. While overall the book is still tilted towards Linux/UNIX because most open source programs are still Linux/UNIX-only, I have tried to put Windows-based security tools in every chapter. I've also included helpful hints and full explanations for those who have never run a UNIX machine.