Internet firewalls are often referred to as secure Internet gateways in other literature. This document uses firewall to refer to a secure Internet gateway.
A firewall, as defined in this document, includes a number of items such as policy, network arrangement, and technical controls and procedures. This document uses firewall system when referring to the hosts or routers that implement the firewall.
This document, when referring to a network protected by a firewall, uses protected subnet or protected LAN (Local Area Network).
Some people dispute whether TCP/IP protocols should be referred to as protocols or services. It could be argued, for example, that TELNET is a protocol, a service, or a command. Where it makes obvious sense, this document uses protocol, otherwise it uses service.
This document uses application gateways to refer to some firewall systems as opposed to bastion hosts.
As much as possible, this document avoids using terms such as hacker and cracker, and uses instead the less ambiguous intruder and attacker.