傳輸控制協(xié)議網(wǎng)際協(xié)議指導(dǎo)(翻譯)

傳輸控制協(xié)議/網(wǎng)際協(xié)議指導(dǎo)本備忘錄的狀態(tài)這本RFC是TCP/IP協(xié)議的指南，重點(diǎn)介紹通過一個(gè)路由器從來源主機(jī)提交一個(gè)IP數(shù)據(jù)包到目的地主機(jī)的步驟。它不指定一個(gè)因特網(wǎng)標(biāo)準(zhǔn)。緒論這本教程僅僅包含TCP/IP技術(shù)的突出特點(diǎn)，因此它是TCP/IP技術(shù)的〃梗概〃。它忽略了TCP/IP發(fā)展的歷史以及導(dǎo)致它廣泛使用的商業(yè)原因，并且忽略它在與ISO（國(guó)際標(biāo)準(zhǔn)化組織）的OSI（開放式系統(tǒng)互連參考模型）相比的發(fā)展前景。實(shí)際上，很多技術(shù)的信息也被忽略。它所講述的是在TCP/IP環(huán)境工作的專業(yè)人員必須理解的最少信息。這些專業(yè)人員包括系統(tǒng)管理員，系統(tǒng)程序員，和網(wǎng)絡(luò)管理員。這本教程使用的例子是在UNIX/TCP/IP環(huán)境下執(zhí)行的，然而主要方面能在所有的TCP/IP環(huán)境下執(zhí)行。這本指南的目的是解釋TCP/IP的基本問題而不是定義它。如果有任何問題關(guān)于協(xié)議的詳細(xì)說明，請(qǐng)參考定義RFC的實(shí)際的標(biāo)準(zhǔn)。下一節(jié)是TCP/IP的概述，然后是各個(gè)組成部分的詳細(xì)描述.TCP/IP概述術(shù)語〃TCP/IP〃通常意味著任何與TCP（傳輸控制協(xié)議）和IP（網(wǎng)際協(xié)議）有聯(lián)系的東西，它包括其它的協(xié)議，應(yīng)用軟件，甚至網(wǎng)絡(luò)媒介。這些協(xié)議的一個(gè)示例是：UDP（用戶數(shù)據(jù)報(bào)協(xié)議），ARP（地址解析協(xié)議），和ICMP（控制報(bào)文協(xié)議）。這些應(yīng)用的一個(gè)示例是:telnet（遠(yuǎn)程登錄），ftp（文件傳遞協(xié)議），和rep。更精確的應(yīng)該說是"因特網(wǎng)技術(shù)"。使用因特網(wǎng)技術(shù)的一個(gè)網(wǎng)絡(luò)被稱為"INTERNET（因特網(wǎng)）〃。2.1基本結(jié)構(gòu)為了理解TCP/IP你必須理解下面的邏輯結(jié)構(gòu)：IIIIIIIIIIIIIIIIIIIIIIIIInetworkItcpIIarpIapplieationsII..\I/...IIIIIIIIIIIIIIIudpIIipI一* IIIIenetI—@一一

—I—EthernetCable（纜線）圖l.TCP/IP的基本結(jié)構(gòu)這是TCP/IP協(xié)議的分層結(jié)構(gòu)在互連網(wǎng)計(jì)算機(jī)上的表示，用互連網(wǎng)技術(shù)互相通信的每臺(tái)計(jì)算機(jī)有這樣的分層結(jié)構(gòu)。這樣的分層結(jié)構(gòu)決定了計(jì)算機(jī)在internet上互相通信的方式。數(shù)據(jù)通過這樣的分層結(jié)構(gòu)從上層傳到底層，然后通過網(wǎng)線把數(shù)據(jù)傳送出去。底層的水平線代表以太網(wǎng)網(wǎng)線，〃0〃代表收發(fā)器，〃*〃代表IP地址，〃@〃代表網(wǎng)址,理解這樣的分層結(jié)構(gòu)式理解INTERNET技術(shù)的基礎(chǔ)。這種分層結(jié)構(gòu)貫川于這本指南的始終。2.2相關(guān)術(shù)語通過一個(gè)internet傳送的數(shù)據(jù)流的名字由它在協(xié)議堆棧的什么地方存在而決定?？傮w上來說：如果它在一個(gè)以太網(wǎng)上，它被稱為以太網(wǎng)幀；如果它在以太網(wǎng)與網(wǎng)際協(xié)議模塊之間，它就被稱為一個(gè)IP包；如果它在網(wǎng)際協(xié)議模塊與UDP模塊之間，它被稱為一個(gè)UDP包；如果它在網(wǎng)際協(xié)議模塊和TCP模塊之間，它被稱為一TCP包（更通常說，傳輸消息包）；并且如果它在網(wǎng)絡(luò)應(yīng)用程序間，它被稱為應(yīng)用消息包。這些定義是不很確切的。不同的版本有不同的定義，要看更精確的定義請(qǐng)參考（RFC1122,section1.3.3.）。驅(qū)動(dòng)程序是直接與網(wǎng)卡接口硬件通信的軟件。應(yīng)用程序模塊是與驅(qū)動(dòng)程序或與網(wǎng)絡(luò)應(yīng)用軟件或另一個(gè)應(yīng)用程序模塊通信的軟件術(shù)語：驅(qū)動(dòng)程序，應(yīng)用程序模塊，以太網(wǎng)幀，TCP包，UDP包，TCP消息，以及應(yīng)用程序消息在此指南中經(jīng)常被使用。2.3.數(shù)據(jù)傳送讓我們看看當(dāng)它經(jīng)過如圖1所示的協(xié)議堆棧流動(dòng)時(shí)的情況，當(dāng)應(yīng)用程序使用TCP（傳輸控制協(xié)議），數(shù)據(jù)在應(yīng)用程序與TCP模塊之間傳遞。當(dāng)應(yīng)用程序使用UDP（用戶數(shù)據(jù)報(bào)協(xié)議），數(shù)據(jù)在應(yīng)用程序與UDP模塊之間傳遞。FTP（文件傳輸協(xié)議）是使用TCP包的典型應(yīng)用。在這個(gè)例子的協(xié)議堆棧是FTP/TCP/IP/ENET。SNMP（簡(jiǎn)單網(wǎng)絡(luò)管理協(xié)議）是使用UDP的應(yīng)用。在這個(gè)例子的協(xié)議堆棧是SNMP/UDP/IP/ENET。ImultiplexerIflowofdataII1I1v1輸出。1 23...n\||/\||/圖2.n—to—1multiplexerTCP模塊，UDP模塊，和以太網(wǎng)驅(qū)動(dòng)程序是n-to-1multiplexers（多路復(fù)用器）。作為multiplexers輸出。1 23...n\||/\||/圖2.n—to—1multiplexer1 23...n\ I I / 八\II/I flowIde-multiplexerIof dataI II I1and1—to—nde-multiplexer如果以太網(wǎng)幀離開網(wǎng)卡進(jìn)入以太網(wǎng)驅(qū)動(dòng)程序，包能向上傳遞給ARP（地址解析協(xié)議）模塊或到IP（網(wǎng)間協(xié)議）模塊。在以太網(wǎng)幀的字段屬性決定了以太網(wǎng)幀是否被傳遞給ARP或IP模塊。如果IP報(bào)文進(jìn)入IP包，它被向上傳遞到TCP或UDP,由在IP頭的字段屬性決定。如果UDP報(bào)文進(jìn)入U(xiǎn)DP，應(yīng)用消息向上傳遞給網(wǎng)絡(luò)應(yīng)用程序，這是由UDP頭的端口值決的。如果TCP消息進(jìn)入TCP，應(yīng)用消息向上傳遞給網(wǎng)絡(luò)應(yīng)用程序，這是由TCP頭的端口值決的。向下復(fù)用很容易實(shí)現(xiàn)因?yàn)閺拿恳粋€(gè)開始點(diǎn)只有一條向下的路徑；每個(gè)協(xié)議模塊增加它的頭信息從而使包能在目的計(jì)算機(jī)上被分開。從應(yīng)用程序出來的數(shù)據(jù)通過TCP或者UDP復(fù)合到IP模塊，然后被送到更低層。盡管因特網(wǎng)技術(shù)支持許多網(wǎng)絡(luò)媒介，在我們這里使用的例子都是以太網(wǎng)來討論的因?yàn)橐蕴W(wǎng)是基于IP的最常見物理網(wǎng)絡(luò)。在圖1中的計(jì)算機(jī)有唯一的以太網(wǎng)連接。6字節(jié)的物理地址對(duì)每一個(gè)在以太網(wǎng)的接口是唯一的，它們被存儲(chǔ)在以太網(wǎng)驅(qū)動(dòng)程序的底層接口中。計(jì)算機(jī)同樣有4字節(jié)的IP地址，這個(gè)地址被存儲(chǔ)在IP模塊的底層，IP地址對(duì)在internet上的每臺(tái)計(jì)算機(jī)必須是唯一的。每臺(tái)計(jì)算機(jī)知道屬于自己的IP地址和物理地址。2.4.兩個(gè)網(wǎng)絡(luò)接口如果一臺(tái)計(jì)算機(jī)和2個(gè)獨(dú)立的以太網(wǎng)連接，它的樣子如圖3所示：networkapplications|I.\I/..\I/.|TCP|IudpI\/IipI—―* *——|ARP|IIIIIarpI\1III/|ENET| |ENET| |――@--——@-- || EthernetCable2 o EthernetCable1圖3.TCP/IPNetworkNodeon2Ethernets請(qǐng)注意這臺(tái)計(jì)算機(jī)有兩個(gè)物理地址和兩個(gè)IP地址。從圖3的結(jié)構(gòu)中我們發(fā)現(xiàn)計(jì)算機(jī)有多于一個(gè)的物理網(wǎng)絡(luò)接口，那么IP模塊就是ntom復(fù)用器和mton分路器的結(jié)合。1 23...n1 23... n\II/I\II /\II/I\II /IflowflowImultiplexerI ofIde-multiplexerI ofdatadata/II\I /II\/II\v /II\1 23...m 1 23...m

圖4.n-to-m復(fù)用器andm-to-n分路器從上面我們可以看出這種多路技術(shù)可以從任意的方向接收和發(fā)送數(shù)據(jù)，有一個(gè)以上網(wǎng)絡(luò)接口的IP模塊比我們最初的把數(shù)據(jù)從一個(gè)網(wǎng)絡(luò)送到另一個(gè)網(wǎng)絡(luò)的例子要復(fù)雜的多，數(shù)據(jù)可以從各個(gè)網(wǎng)絡(luò)接口傳過來也可被送向網(wǎng)絡(luò)。TCP UDPTOC\o"1-5"\h\z\ /\/I IP II II — II / \II/vI

/\/ \data datacomesin goesouthere here圖5.IP包的發(fā)送和接收的例子發(fā)送IP包到另一個(gè)網(wǎng)絡(luò)的過程叫做傳遞IP包，一臺(tái)專門用來傳遞IP包的計(jì)算機(jī)。計(jì)算機(jī)叫做〃路由器〃。就如你從圖中看到的一樣，在路由器上傳遞的IP包不涉及TCP和UDP模塊,一些路由器執(zhí)行時(shí)更本就不要TCP或UDP模塊。2.5.IP產(chǎn)生專一的邏輯網(wǎng)絡(luò)IP模塊是interner技術(shù)成功的中心所在，當(dāng)消息向下通過協(xié)議棧時(shí)每一個(gè)模塊或驅(qū)動(dòng)程序加上自己的頭到消息中去。每一個(gè)模塊或驅(qū)動(dòng)程序從消息中去掉相應(yīng)的頭當(dāng)消息沿協(xié)議棧向上傳時(shí)，IP頭包含了用以從許多物理網(wǎng)絡(luò)中區(qū)分唯一的邏輯網(wǎng)絡(luò)的IP地址，互相連接的物理網(wǎng)絡(luò)是internet的組成部分，這些互相聯(lián)絡(luò)的物理網(wǎng)絡(luò)就構(gòu)成了internet。獨(dú)立的物理網(wǎng)絡(luò)IP通過網(wǎng)絡(luò)應(yīng)用程序處于網(wǎng)絡(luò)硬件之中，如果你發(fā)明新的物理網(wǎng)絡(luò)，你可以投入使用只要你用新的驅(qū)動(dòng)程序基于IP。因此，盡管硬件技術(shù)在變化網(wǎng)絡(luò)應(yīng)用程序仍舊是完美無缺以及不易受攻擊的?；ハ嗤ㄐ湃绻麅膳_(tái)計(jì)算機(jī)能夠互相通信，就稱它們是"能互操作的"。如果internet暢通，就稱它們有〃互用性〃。有多方面用途計(jì)算機(jī)的用戶從互連網(wǎng)獲益因?yàn)樵谑袌?chǎng)上有的協(xié)同工作能力的計(jì)算機(jī)。一般地，當(dāng)你買了一臺(tái)計(jì)算機(jī)，它就能互相通信。如果計(jì)算機(jī)沒有互操作性，且互操作性不能加上，則這樣地計(jì)算機(jī)就沒有市場(chǎng)競(jìng)爭(zhēng)力。以太網(wǎng)這一節(jié)是以太網(wǎng)技術(shù)的簡(jiǎn)短復(fù)習(xí)。一個(gè)以太網(wǎng)幀包含目的地地址，源地址，屬性域，和數(shù)據(jù)。物理地址是6字節(jié)。每一個(gè)裝置有它自己的物理地址和監(jiān)聽物理幀中的目的地址。所有的裝置用通配符"FF-FF-FF-FF-FF-FF"（十六進(jìn)制）的方式監(jiān)聽目的地址，被稱為〃廣播〃地址。以太網(wǎng)使用CSMA/CD（帶有檢測(cè)沖突的載波偵聽多路存取）。CSMA/CD意味著所有裝置在同一個(gè)媒介中通信，在某一個(gè)時(shí)刻只能有一個(gè)在傳送數(shù)據(jù)，它們能同時(shí)接收數(shù)據(jù)。如果2個(gè)裝置在同一個(gè)傳送數(shù)據(jù)，那么傳送沖突被發(fā)覺，在再次發(fā)送之前兩個(gè)裝置等隨機(jī)（但是很短）的一段時(shí)間。一個(gè)類比一個(gè)很好的說明以太網(wǎng)技術(shù)的類比：一群人在很小道'很黑的屋里聊天。在這個(gè)類比中：物理的網(wǎng)絡(luò)媒介是空氣中的聲波而不是在同一根同軸電纜中的電信號(hào)每一個(gè)人能聽見其它人的說話（信號(hào)感覺），每一個(gè)人有相同的權(quán)利說話（多種可能），但是沒有人會(huì)發(fā)表長(zhǎng)篇大論因?yàn)樗麄兌Y貌。如果有人不懂禮貌，他被要求離開房間（也就是，被拋離網(wǎng)絡(luò)）。沒有人會(huì)說話當(dāng)有人說話時(shí)，但是當(dāng)兩個(gè)人同時(shí)開始說話，他們會(huì)立即知道因?yàn)樗麄兟牭揭恍┧麄冏约簺]有說過的話（沖突檢出）。當(dāng)兩個(gè)人意識(shí)到時(shí)，他們等一會(huì)，然后一人開始說話。另一個(gè)在開始自己的說話之前聽著別人說一直等著第一個(gè)人說完。每一個(gè)人有唯一的名字（唯一的物理地址）以避免混亂。某一個(gè)時(shí)間一人說話，他會(huì)先通知他將要交談的人以及自己的名字（以太網(wǎng)目的地址，源地址，分別的），也就是："Hello,thisisjack,..blahblahblah..",如果他想和所有的人交談，他會(huì)說"everyone"（廣播地址），也就是說："hello,everyone,thisisjack,..blahblahblah.."。

ATCP/IPTutorialStatusofthisMemoThisRFCisatutorialontheTCPIPprotocolsuite,focusingparticularlyonthestepsinforwardinganIPdatagramfromsourcehosttodestinationhostthrougharouter.ItdoesnotspecifyanInternetstandard.Distributionofthismemoisunlimited.1.IntroductionThistutorialcontainsonlyoneviewofthesalientpointsofTCP/IP,andthereforeitisthebarebonesofTCP/IPtechnology.Itomitsthehistoryofdevelopmentandfunding,thebusinesscaseforitsuse,anditsfutureascomparedtoISOOSI.Indeed,agreatdealoftechnicalinformationisalsoomitted.WhatremainsisaminimumofinformationthatmustbeunderstoodbytheprofessionalworkinginaTCP/IPenvironment.Theseprofessionalsincludethesystemsadministrator,thesystemsprogrammer,andthenetworkmanager.ThistutorialusesexamplesfromtheUNIXTCP/IPenvironment,howeverthemainpointsapplyacrossallimplementationsofTCP/IP.Notethatthepurposeofthismemoisexplanation,notdefinition.Ifanyquestionarisesaboutthecorrectspecificationofaprotocol,pleaserefertotheactualstandardsdefiningRFC.ThenextsectionisanoverviewofTCP/IP,followedbydetaileddescriptionsofindividualcomponents.2.TCP/IPOverviewThegenerictermTCP/IPusuallymeansanythingandeverythingrelatedtothespecificprotocolsofTCPandIP.Itcanincludeotherprotocols,applications,andeventhenetworkmedium.AsampleoftheseprotocolsareUDP,ARP,andICMP.AsampleoftheseapplicationsareTELNET,FTP,andrcp.Amoreaccuratetermisinternettechnology.Anetworkthatusesinternettechnologyiscalledaninternet.BasicStructureTounderstandthistechnologyyoumustfirstunderstandthefollowinglogicalstructure||||||||||||\|/..\|/...|

——|

|TCP| |UDP||——|\/| ||IP| |

| —* ||||TOC\o"1-5"\h\z| |ARP| |||||I ― II \ II——| |ENET| || ——@— | | I

o EthernetCableFigurel.BasicTCP/IPNetworkNodeThisisthelogicalstructureofthelayeredprotocolsinsideacomputeronaninternet.Eachcomputerthatcancommunicateusinginternettechnologyhassuchalogicalstructure.Itisthislogicalstructurethatdeterminesthebehaviorofthecomputerontheinternet.Theboxesrepresentprocessingofthedataasitpassesthroughthecomputer,andthelinesconnectingboxesshowthepathofdata.ThehorizontallineatthebottomrepresentstheEthernetcable;theoisthetransceiver.TheistheIPaddressandthe@istheEthernetaddress.Understandingthislogicalstructureisessentialtounderstandinginternettechnology;itisreferredtothroughoutthistutorial.TerminologyThenameofaunitofdatathatflowsthroughaninternetisdependentuponwhereitexistsintheprotocolstack.InsummaryifitisonanEthernetitiscalledanEthernetframe;ifitisbetweentheEthernetdriverandtheIPmoduleitiscalledaIPpacket;ifitisbetweentheIPmoduleandtheUDPmoduleitiscalledaUDPdatagram;ifitisbetweentheIPmoduleandtheTCPmoduleitiscalledaTCPsegment(moregenerally,atransportmessage);andifitisinanetworkapplicationitiscalledaapplicationmessage.Thesedefinitionsareimperfect.Actualdefinitionsvaryfromonepublicationtothenext.MorespecificdefinitionscanbefoundinRFC1122,section1.3.3.Adriverissoftwarethatcommunicatesdirectlywiththenetworkinterfacehardware.Amoduleissoftwarethatcommunicateswithadriver,withnetworkapplications,orwithanothermodule.Thetermsdriver,module,Ethernetframe,IPpacket,UDPdatagram,TCPmessage,andapplicationmessageareusedwhereappropriatethroughoutthistutorial.FlowofDataLet'sfollowthedataasitflowsdownthroughtheprotocolstackshowninFigure1.ForanapplicationthatusesTCP(TransmissionControlProtocol),datapassesbetweentheapplicationandtheTCPmodule.For

applicationsthatuseUDP(UserDatagramProtocol),datapassesbetweentheapplicationandtheUDPmodule.FTP(FileTransferProtocol)isatypicalapplicationthatusesTCP.ItsprotocolstackinthisexampleisFTP/TCP/IP/ENET.SNMP(SimpleNetworkManagementProtocol) isanapplicationthatusesUDP.ItsprotocolstackinthisexampleisSNMPUDPIPENET.TheTCPmodule,UDPmodule,andtheEthernetdriveraren—to—1multiplexers.Asmultiplexerstheyswitchmanyinputstooneoutput.Theyarealso1—to—nde—multiplexers.Asde—multiplexerstheyswitchtheprotocoloneinputheader.tomanyoutputsaccordingtothetypeTheTCPmodule,UDPmodule,andtheEthernetdriveraren—to—1multiplexers.Asmultiplexerstheyswitchmanyinputstooneoutput.Theyarealso1—to—nde—multiplexers.Asde—multiplexerstheyswitchtheprotocoloneinputheader.tomanyoutputsaccordingtothetypefieldin3...|3...|||/Figure2.n—to—1EthernetframecomesupintotheEthernetdriveroffthe|Figure2.n—to—1EthernetframecomesupintotheEthernetdriveroffthe|multiplexer|flowofdata1|de—multiplexer|flowofdata11111v111multiplexerand1—to—nde-multiplexerIfannetwork,thepacketcanbepassedupwardstoeithertheARP(AddressResolutionProtocol)moduleortotheIP(InternetProtocol)module.ThevalueofthetypefieldintheEthernetframedetermineswhethertheEthernetframeispassedtotheARPortheIPmodule.IfanIPpacketcomesupintoIP,theunitofdataispassedupwardstoeitherTCPorUDP,asdeterminedbythevalueoftheprotocolfieldintheIPheader.IftheUDPdatagramcomesupintoUDP,theapplicationmessageispassedupwardstothenetworkapplicationbasedonthevalueoftheportfieldintheUDPheader.IftheTCPmessagecomesupintoTCP,theapplicationmessageispassedupwardstothenetworkapplicationbasedonthevalueoftheportfieldintheTCPheader.Thedownwardsmultiplexingissimpletoperformbecausefromeachstartingpointthereisonlytheonedownwardpath;eachprotocolmoduleaddsitsheaderinformationsothepacketcanbede—multiplexedatthedestinationcomputer.DatapassingoutfromtheapplicationsthrougheitherTCPorUDPconvergesontheIPmoduleandissentdownwardsthroughthelowernetworkinterfacedriver.Althoughinternettechnologysupportsmanydifferentnetworkmedia,EthernetisusedforallexamplesinthistutorialbecauseitisthemostcommonphysicalnetworkusedunderIP.ThecomputerinFigure1

hasasingleEthernetconnection.The6-byteEthernetaddressisuniqueforeachinterfaceonanEthernetandislocatedatthelowerinterfaceoftheEthernetdriver.Thecomputeralsohasa4—byteIPaddress.ThisaddressislocatedatthelowerinterfacetotheIPmodule.TheIPaddressmustbeuniqueforaninternet.ArunningcomputeralwaysknowsitsownIPaddressandEthernetaddress.TwoNetworkInterfacesIfacomputerisconnectedto2separateEthernetsitisasinFigurenetworkapplications|networkI..\I/...IItcpI―I

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/ I— II IIPI II ?* ?* II IarpI I I IarpIII — I I —II \ I I /II II IenetI IenetIII ——@--——@--II EthernetCable2 o EthernetCable1Figure3.TCP/IPNetworkNodeon2EthernetsPleasenotethatthiscomputerhas2Ethernetaddressesand2IPaddresses.Itisseenfromthisstructurethatforcomputerswithmorethanonephysicalnetworkinterface,theIPmoduleisbothan-to-mmultiplexerandanm-to-nde-multiplexer.1 23... n1 23... n\II /I\II /\II / I\II /flowflowImultiplexerI ofIde-multiplexerI ofdatadata/II\I /II\I/II \V /II \ I1 23...m 1 23...mFigure4.n—to—MMultiplexerandM—to—nde-MULtiplexerItperformsthisMultiplexingineitherdirectiontoaccoMModateincoMingandoutgoingdata.AnIPModulewithMorethan1networkinterfaceisMorecomplexthanouroriginalexampleinthatitcanforwarddataontothenextnetwork.Datacanarriveonanynetworkinterfaceandbesentoutonanyother.TCP UDP\ /\/IPII—II/ \II/v|/\

/ \data datacomesin goesouthere hereFigure5.ExampleofIPForwardingaIPPacketTheprocessofsendinganIPpacketoutontoanothernetworkiscalledforwardinganIPpacket.AcomputerthathasbeendedicatedtothetaskofforwardingIPpacketsiscalledanIP—router.Asyoucanseefromthefigure,theforwardedIPpacketnevertouchestheTCPandUDPmodulesontheIP—router.SomeIP—routerimplementationsdonothaveaTCPorUDPmodule.IPCreatesaSingleLogicalNetworkTheIPmoduleiscentraltothesuccessofinternettechnology.Eachmoduleordriveraddsitsheadertothemessageasthemessagepassesdownthroughtheprotocolstack.Eachmoduleordriverstripsthecorrespondingheaderfromthemessageasthemessageclimbstheprotocolstackuptowardstheapplication.TheIPheadercontainstheIPaddress,whichbuildsasinglelogicalnetworkfrommultipiephysicalnetworks.Thisinterconnectionofphysicalnetworksisthesourceofthenameinternet.AsetofinterconnectedphysicalnetworksthatlimittherangeofanIPpacketiscalledaninternet.PhysicalNetworkIndependenceIPhidestheunderlyingnetworkhardwarefromthenetworkapplications.Ifyouinventanewphysicalnetwork,youcanputitintoservicebyimplementinganewdriverthatconnectstotheinternetunderneathIP.Thus,thenetworkapplicationsremainintactandarenotvulnerabletochangesinhardwaretechnology.Interoperab訂ityIftwocomputersonaninternetcancommunicate,theyaresaidtointeroperate;ifanimplementationofinternettechnologyisgood,itissaidtohaveinteroperability.Usersofgeneral-purposecomputersbenefitfromtheinstallationofaninternetbecauseoftheinteroperabilityincomputersonthemarket.Generally,whenyoubuyacomputer,itwillinteroperate.Ifthecomputerdoesnothaveinteroperability,andinteroperabilitycannotbeadded,itoccupiesarareandspecialnicheinthemarket.3.EthernetThissectionisashortreviewofEthernettechnology.AnEthernetframecontainsthedestinationaddress,sourceaddress,typefield,anddata.AnEthernetaddressis6bytes.EverydevicehasitsownEthernetaddressandlistensforEthernetframeswiththatdestinationaddress.AlldevicesalsolistenforEthernetframeswithawild-carddestinationaddressofFF-FF-FF-FF-FF-FF(inhexadecimal),calledabroadcastaddress.EthernetusesCSMACD(CarrierSenseandMultipleAccesswithCollisionDetection).CSMACDmeanstha