¹³For the curious, iptables is an administration-level utility application for Linux operating systems, used to edit the ACL rulebase of the operating system’s built-in software firewall. Each line of text in these examples is a command that may be typed manually at the command-line interface of the operating system, or more commonly written to a script file to be automatically read and executed upon start-up of the computer. The -A option instructs iptables to Append a new rule to the ACL. These rules are organized into groups called “chains” which are given names such as INPUT and OUTPUT. While the specific format of ACL rules are unique to each firewall, they share many common features.