The key to implementing a network security policy is to understand that a firewall is simply a technical tool that reflects and enforces a network security policy for accessing network resources.
A rule base is an ordered set of individual network security rules, against which each attempted connection is checked. Each rule specifies the source, destination, service, and action to be taken for each connection. A rule also specifies how a communication is tracked, logged, and displayed. In other words, the rule base is the implementation of the security policy.