Encyclopedia of Computer Science and Technology

196 firewallcontrolled by a state table. The interaction of such automatacan produce astonishingly complex patterns (see cellularautomata).ApplicationsMany programs and operating systems are structured as anendless loop where an input (or command) is processed,the results returned, the next input is processed, and soon, until an exit command is received. A mode or state canbe used to determine the system’s activity. For example, aprogram might be in different modes such as waiting forinput, processing input, displaying results, and so on. Theprogram logic will refer to the current state to determinewhat to do next and at some point the logic will transitionthe system to the next state in the sequence. The validity ofsome kinds of programs, protocols, or circuits can thereforebe proven by showing that there is an equivalent finite-statemachine—and thus that all possible combinations of inputshave been accounted for.Finite-state machines have many other interesting applications.Simple organisms can be modeled as a set of statesthat interact with the environment (see artificial life).The lower-level functions of robots can also be representedas a set of interacting finite-state machines. Even videogame characters often use FSMs to give them a repertoire ofplausible behavior.Further ReadingFinite State Machine Editor [software]. Available online. URL:http://fsme.sourceforge.net/. Accessed August 4, 2007.Meyer, Nathaniel. “Finite State Machine Tutorial.” Availableonline. URL: http://www.generation5.org/content/2003/FSM_Tutorial.asp. Accessed August 4, 2007.Wagner, Ferdinand, et al. Modeling Software with Finite StateMachines: A Practical Approach. Boca Raton, Fla.: CRC Press,2006.This diagram shows a finite-state representation of a ZIP code. Thearrows link each state (within a circle) to its possible successor. Inthis simple example each digit must be followed by another digituntil the fifth digit, which can either be followed by a blank (indicatinga five-digit ZIP code) or four more digits for a 9-digit ZIP.result in a specified output. That output becomes the newstate of the machine. (Alternatively, the machine can beset so that only the current state determines the output,without regard to the previous state.) This is supportedby the underlying structure of the logic switching withincomputer circuits as well as the “statefulness” of all calculations.(Given n, n+1 is defined, and so on.) Alan Turingshowed that combining the state mechanism with aninfinite memory (conceptualized as an endless roll of tape)amounted to a universal computer—that is, a mechanismthat could perform any valid calculation, given enough time(see Turing, Alan).The idea of the sequential (or state) machine is closelyrelated to automata, which are entities whose behavior isfirewallThe vulnerability of computer systems to malicious orcriminal attack has been greatly increased by the growingnumber of connections between computers (and local networks)and the worldwide Internet (see computer crimeand security, Internet, and tcp/ip). The widespread useof permanent broadband connections by consumers (suchas DSL and cable modem links) has increased the risk tohome users. Intruders can use “port scanning” programsto determine what connections a given system or networkhas open, and can use other programs to snoop and steal ordestroy sensitive data.A firewall is a program (or combination of software andhardware) that sits between a computer (or local network)and the Internet. Typical firewall functions include:• Examining incoming data packets and blocking thosethat include commands to examine or use unauthorizedports or IP addresses• Blocking data packets that are associated with commonhacking techniques such as “trojans” or “backdoor”exploitations