Encyclopedia of Computer Science and Technology

flag 197• Hiding all the internal network addresses on a localnetwork, presenting only a single address to theoutside world (this is also called NAT, or NetworkAddress Translation)• monitoring particular applications such as ftp (filetransfer protocol) and telnet (remote login), restrictingthem to certain addresses. Often a special addresscalled a proxy is established rather than allowingdirect connections between the outside and the localnetwork.Firewalls are usually configured by providing a rule thatspecifies what is to be done based on the origin address orother characteristics of an incoming packet. Because connectionsmade by local programs to the outside can alsocompromise the system, rules are also created for suchapplications. The firewall package may come with a set ofdefault rules for common applications and situations. Whensomething not covered by the rules happens, the user willbe prompted and guided to establish a new rule.Modern firewalls are “stateful,” meaning that they keeptrack not only of the source and destination of individualpackets but their context (including originating application).Microsoft Windows Vista has improved the operatingsystem’s built-in firewall, at the expense of added complexity.Zone Labs’s ZoneAlarm is another popular PC firewall.Linux provides a default firewall called iptables, which canbe configured by a variety of applications. For added protection,users of broadband Internet connections should notconnect their PC directly to the Internet. Rather, an inexpensivewired or wireless router that includes a built-infirewall can be connected on one side to the cable or DSLmodem and on the other side to one or more computers inthe local network.Internet security packages for home users often combinea firewall with other services such as virus protection,parental control, and blocking of objectionable content oradvertising.Further ReadingHome PC Firewall Guide. Available online. URL: http://www.firewallguide.com/. Accessed August 4, 2007.Komar, Brian, Ronald Beekelaar, and Joern Wettern. Firewalls forDummies. 2nd ed. New York: Wiley, 2003.Noonan, Wes, and Ido Dubrawsky. Firewall Fundamentals. Indianapolis:Cisco Press, 2006.ZoneAlarm. Available online. URL: http://www.zonealarm.com.Accessed August 4, 2007.Zwicky, Elizabeth D., Simon Cooper, and D. Brent Chapman.Building Internet Firewalls. 2nd ed. Sebastapol, Calif.: O’ReillyMedia, 2000.FireWireFireWire is a high-speed serial interface used by personalcomputers and digital audio and video equipment. (Thename FireWire is an Apple brand name, but it is used generically.Technically it is the IEEE 1394 Serial Bus.)FireWire was developed in the 1990s by the IEEE P1394Working Group with substantial funding from Apple andhelp from engineers from major corporations includingIBM, Digital Equipment Corporation (DEC), Sony, andTexas Instruments. In 1993 it was hailed as the “most significantnew technology” by Byte magazine.FireWire was intended to replace Apple’s parallel SCSI(Small Computer System Interface). (Sony’s implementation,called I.Link, omits the two power pins in favor of aseparate power connector.) However, because Apple askedfor $1.00 per port in patent royalties, Intel instead developeda faster version of the universal serial bus (see usb)and that, rather than FireWire, is the standard port on mostWindows machines.Common uses for FireWire include connecting digitalvideo (such as camcorder) devices, audio devices, and somedata storage devices. FireWire is favored over USB 2.0 formany professional applications because of its higher speedand power distribution capabilities. However, it is moreexpensive than USB 2.0, which provides sufficient speedfor many consumer peripherals such as digital cameras andprinters.Further ReadingAnderson, Don, and MindShare, Inc. FireWire System Architecture.2nd ed. Boston: Addison-Wesley Pearson Education, 1998.FireWire (Apple Developer Connection). Available online. URL:http://developer.apple.com/hardwaredrivers/firewire/index.html. Accessed September 20, 2007.flagA flag is a variable that is used to specify a particular conditionor status (see variable). Usually a flag is either true orfalse. For example, a flag Valid_Form could be set to truebefore the input form is processed. If the validation checkfor any data field fails, the flag would be set to false. Afterthe input procedure has ended, the main program wouldcheck the Valid_Form flag. If it’s true, the data on the formis processed (for example, continuing on to the paymentprocess). If the flag is false, the input form might be redisplayedwith errors or omissions highlighted.Flags can be combined to check multiple conditions. Forexample, suppose the input form routine also looked up thecustomer’s account and checked to make sure the customerwas approved for purchasing. The test for this might read:If Valid_Form and Valid_Customer then// continue processing else// display error messagesIn such cases, the flags are combined using the appropriateand or or operators (see Boolean operators).While flags are often used inside a routine to keep trackof processing, modern programming practice discouragesthe use of “global” flags at the top level of the program. Aswith other global variables, such flags are vulnerable tobeing unpredictably changed or to having two parts of theprogram check the same flag without being able to rely onits state. (Thus a routine relies on a global flag being truebut calls another routine that sets the flag to false withoutthe original routine checking it again.) If several routines