Overview of the Book The rest of this book proceeds as follows. In Chapter 1.4, we introduce the method of structured firewall design. In Chapter 2.8, we present the method of diverse firewall design. In Chapter 3.4, we show a model for specifying stateful firewalls and some method for analyzing the the properties of stateful firewalls specified in this model. In Chapter 4.5.2, we present how to describe and process firewall queries. In Chapter 5.5, we develop theorems and algorithms for removing all the redundant rules in any given firewall. Finally, in Chapter 6.4, we summarize our research and suggest several topics for future research.