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《电子与通信技术专业英语 第3版 教学课件 2 作者 刘小芹 刘骋 Unit Four.ppt》由会员上传分享，免费在线阅读，更多相关内容在应用文档-天天文库。
1、UnitFourTextATextB能力分析训练TextAThefirewallactuallydefinesthedifferencebetweenInternetandIntranet.Theyhavetodowithperformancemanagementandprioritizationofservice,buttheprimaryoneisthefirewall.ItisthedifferencebetweenInternetandIntranet.Firewallsareveryimportant.But,isthefi
2、rewalltheonlywaytoconnectfromyourIntranet,fromyourinsidenetwork,totheInternet?Inmostcases,theanswerisno.Firewallsareimperfectbecausetheyprovideonlyasinglepointatwhichwecanissuesomecontrol.Therearemanykindsoffirewalls,suchas“simpletrafficrecordingsystem”,“IPpacketsfilter
3、gateway”(orIPpacketsexaminerouter)andthe“proxyapplicationgateway”.Eachprovidesdifferentlevelofsecuritychecking.Whichisthebest?Itishardtosay,becauseitdependsonwhatkindoffiles(database)youwanttoprotectandwhatkindofprotectionyouwantthefirewalltoprovide,sinceweknowthatthere
4、arefourkindsofdocuments:publicused,copyrighted,secretandconfidential.Amongallthefirewalls,“simpletrafficrecordingsystem”isthesimplestone.Itisakindofrecordingdevicethatcankeepalltherecordsofpassingthroughtrafficinadatabaseordocumentforauditingpurpose.herestkindsoffirewal
5、lsareallstructuredonthebaseofpacketsfilteringtechniques.Thepacketsaresmallsubsetsofdatathattravelthroughnetworks,eachwiththesourceaddress,destinationaddress,protocolused,andpartoftheE-mailtexts.Thiskindoffirewallshastwofunctions:chokesandgates.Therearetwotypesofrulesuse
6、dforexaminingthedatatraffic:oneiscalled“defaultpermit”,whichallowsalltrafficexceptthatexplicitlyblockedbyfirewalladministrators;theotheriscalled“defaultdeny”,whichdeniesalltrafficexceptthatexplicitlyallowedbythefirewalladministrator.Generallyspeaking,thelatterisbetter.T
7、hepacketsfiltergate(orpacketsexaminerouter)isamorecommonusedfirewall.Itisakindofrouterusedforpacketfiltering.Anotherkindoffirewalliscalled“proxyapplicationgateway”,whichemploystheproxyservicesoftware,whichnotonlyprovidestrafficfilteringatIPandtransportlevels,butalsoprov
8、idesapplicationlevelprotocolcheckingatapplicationlevel.Notes1．WhenwetalkabouttheInternet,wehavetobeverycareful
