1、<p><b>　　附錄3</b></p><p><b>　　Internet</b></p><p>　　Internet，也叫做Net，是世界上最大的計算機網絡。你也許會問：“什么是網絡？”計算機網絡，簡單地說，就是通過某種方法被連在一起的若干計算機。從概念上講，就像若干無線電臺或電視臺連成無電線網絡或電視網絡一樣。</p&

2、gt;<p>　　某些計算機網絡包括一個中心計算機和若干與中心計算機相聯(lián)的遠端的計算機。例如．一臺飛機訂票系統(tǒng)的中心計算機，就與各個機場和旅行社的上萬臺終端相連。其它類型的計算機網絡，包括Internet,允許網絡中每臺計算機與另一臺計算機通信。</p><p>　　Internet可能是世界上最開放的網絡。任何人，只要聯(lián)入Net，就能利用上萬臺計算機提供的服務。這種情況是不多見的，很多網絡在允許用

3、戶使用方面限制性較強，并且對每項服務均有特別安排的口令。盡管有一些服務需要繳費，但絕大多數Internet的服務都是只要你連通了就可免費享受。如果你通過公司、學校還不能進人Internet，你可以付費給Internet訪問權提供商來進入Internet。</p><p>　　怎樣才能連通Internet呢？這就要依你的情況而定，涉及到你的工作或者你的學校。Internet連網的工作可能比較簡單，也可能比較復雜。&

4、lt;/p><p><b>　?。╨）調制解調器</b></p><p>　　Modem是一個小裝置，它使用普通電話線就能將數據從一臺計算機傳到另一臺計算機。這個小巧的盒子在你的計算機和電話線間，在電話線與另一端的計算機間發(fā)揮作用。有的Modem是裝在計算機內的，所以當你不能肯定計算機中是否裝有Modem時，請詢問一下。</p><p>　　如果你

5、需要買一個modem，可選擇最便宜的14400bps的傳真modem。你也可花錢買到速度更快的，但是你現在用的普通電話線不能實現這種快的速度。要弄清楚你的modem是否配有適合你的計算機和操作系統(tǒng)版本的通訊軟件。</p><p>　　利用Internet可做許多有趣的事情，但是你要通過學?；騿挝坏玫綄nternet的訪問權。提供Internet訪問權（通常要收費）的人被稱為Internet服務提供尚(ISP)。

6、</p><p><b>　?。?）聯(lián)機服務</b></p><p>　　聯(lián)機服務是由一臺計算機或由相互聯(lián)系的幾臺計算機提供的服務，包括（但不限于）計算機存取款、購物、約會、娛樂及學習。這種服務可能與Internet有關，也可能無關，例如CompuServe，它就是不依靠Internet而獨立存在的，它允許CompuServe用戶相互之間寄送電子郵件，但不能與Comp

7、uServe以外的用戶聯(lián)系?，F在，許多這種服務已打破局限，用戶可以進行電子郵件寄送．但這與實際的Internet訪問功能還有較大差距。</p><p>　?。?）公告牌（BBS)</p><p>　　電子公告牌系統(tǒng)（簡稱BBS），通常在一個較小范圍內提供聯(lián)機服務。公告牌系統(tǒng)多是區(qū)域性的，費用較低廉，有時甚至對用戶免費。它可能提供電子郵件、交談（用戶之間的聯(lián)機對話）、專題講座、游戲、廣告等許

8、多方面的服務。</p><p><b>　　網關</b></p><p>　　第一個問題是，你的PC機與Internet是由本機直接連接還是通過其它網絡連接。如果是本機連接，那么你的PC機也要運行和Internet一樣采用的TCP/IP網絡協(xié)議的網絡軟件。</p><p>　　另一種情況是，你的PC機運行其它類型的網絡軟件（多數是Novell的

9、Netware，并與一個網關系統(tǒng)連接，一邊采用Netware，另一邊采用TCP/IP還有可能是在一臺PC機里既裝有TCP/IP也裝有其它網絡軟件，同時在同一網絡電纜上運行。</p><p><b>　　以太令牌</b></p><p>　　假定你的PC機自己運行TCP/IP而不通過其它網絡，怎樣與該網絡的其余部分連接呢？有三個選擇：Ethernet（以太網）、Toke

10、n Ring（令牌環(huán)）和Serial line（串行線）。PC機與網絡相連最常用的是Ethernet，這是一種快速而經濟的網絡， 但距離限為一英里以下（對大—些的網絡來說，可用“橋接器”和“路由器”將幾個Ethernet網絡連在一塊）。</p><p><b>　　令牌環(huán)</b></p><p>　　幾年前，IBM公司決定：由于世界上其它計算機廠商都在使用Ether

11、net，它推出名為“令牌環(huán)”網絡，以避免過度兼容帶來的麻煩。這引來了許多技術方面的爭論，對Ethernet和Taken line各自的優(yōu)勢莫衷一是。我們認為，二者的區(qū)別95%都是政策性的。令牌環(huán)網采用了更理想的矩形接頭，想斷掉時就拔出，比Ethernet的許多版本都強。</p><p>　　有數以百萬計的計算機與Internet相連。怎樣找到你需要的那一臺計算機呢？網上的每臺機器均有編號和名稱為標記。和電話號碼一

12、樣，網上的每臺機器（稱作主機）都編有號碼以區(qū)別于其它主機。編號由四部分組成，如123.45.67.89。</p><p>　　大多數主機都有名稱，相對編號來說較易記憶，名稱各部分由點分開。（如Chico.iecc.com）。有些主機有多個名稱，你使用哪個都沒關系。 Internet的每個網絡均規(guī)定了它允許的網絡通訊種類（電子郵件、終端會話及其它連接），你應該掌握你所使用的網絡（一個或多個）的規(guī)則，避免網絡管理者對

13、你發(fā)火。</p><p>　　計算機不計類型，不計大小，只要與Internet相連就被稱作主機。有些主機是大型機或巨型機，為上萬用戶提供服務；有的是小型工作站或PC機，只有一個用戶；有的是專用計算機，如路由器（它連接兩個網絡）、終端服務器。所有這些機器，從Internet的角度看都是主機。</p><p>　　每個機器都分配了一個主機編號，就好像電話號碼一樣。號碼由32位二進制數組成，例如

14、,</p><p>　　10001100101110100101000100000001</p><p>　　這不太好記。為使它較易記憶,將它拆成四個8位組，每組翻譯成一個十進制數字,就變成了：</p><p>　　140.186.81.1</p><p>　　Internet主機號也分為兩個部分：第一部分是網絡編號，第二部分是局域部分，作為

15、主機在具體網絡上的編號。計算機的編號140.186.81.1表明網絡號是140.81，局域主機號（在該網絡上）是81.1。</p><p>　　因為某些網絡擁有許多主機，因而網絡分為三類：大網絡、中網絡、小網絡。在大網絡（A類），中前四位數是網絡號，后三位數是局域部分；在中網絡（B類）中前兩位數是網絡號，后兩位數是局域部分；在小網絡（C類）中，前三位數是網絡號．最后是局域部分。</p><p&

16、gt;　　前四位數就可表明網絡是屬于哪一類。通常，只有一個網絡編號的機構希望將其計算機對內設在多個網絡上。例如，某一部門的所有計算機通常都連于同一個網絡，并由某種連接將各部門網絡連接起來。但要堅守Internet最初的建立原則，有25個網絡的單位必須有25個不同的網絡編號．</p><p>　　這意味著該公司每設立一個新的內部網絡，就得申請—個新的網絡編號。更麻煩的是，Internet中的其它成員就要在表中記下其

17、編號．以便發(fā)送信息時使用。</p><p>　　很顯然，應當采取措施，于是使產生了子網，即一個網絡被分成幾個部分。在子網上，本應是主機編號變成了網絡號。例如網絡140.186中，主機號中第三個數是子網號，對于計算機140.186.81.1，子網號是140.186.81，主機號是1。這樣一來，就可以安裝大量的局域網（可設立的254個子網中只用90個），而外界Internet用戶只需記下140.186就行了。<

18、/p><p>　　實際上，除了最小的網絡外都有子網。</p><p>　　少數機構遇到了相反的問題。它們在C類網上有太多的計算機，多干254臺，又無法達到B類的數目。這時，該機構可以獲取一不分相鄰的網絡編號，將部分網絡號當作主機號，這種方法稱為“跨網”。</p><p>　　一般人都使用姓名，不用編號，因此Internet主機也多以名稱相稱，而不用編號代表。為避免取名稱

19、的困難．便產生了多部分命名法，即域名系統(tǒng)（DNS），主機名是由點隔開的字符串。名稱的最右邊部分是區(qū)。區(qū)名多數是根據機構的性質用三個字母來表示，如下表：</p><p>　　兩個字母的區(qū)名是按地理位置排列的，每個區(qū)對應一個國家或其它政治團體。例如，加拿大的國家代碼是CA，因此加拿大約克大學的一個網址為nexus.yorku.ca。如果你只有一兩臺機器，用地理性名稱較簡便。如果擁有臺數較多，用機構性名稱較容易。這樣你

20、可以自己管理本機構的名稱。</p><p>　　在主機之間傳送數據包時，使用的規(guī)定稱為Internet協(xié)議，即IP。最有名的兩個協(xié)議是：傳輸控制協(xié)議和用戶數據協(xié)議（UDP）。TCP應用很廣，很多人都知道Internet應用程序使用的是TCP與IP的組合——TCP/IP。</p><p>　　在網絡之間傳輸數據包有三種途徑:網橋、路由器和網關。網橋能將兩個網絡連接，變成一個更大的網絡。它最常

21、用于連接兩個Ethernet局域網。網橋監(jiān)視在每個網絡上傳送的所有數據包，當它發(fā)現一個網絡上的包到另一網上的主機時，就將其傳送過去。網橋之所以能分辨哪臺主機在哪個網上。是通過每個網絡的包的返回地址為基礎建立一張大表，列出各個網上的主機。網橋的優(yōu)點是它工作透明。一個網橋能同時處理大量的網絡通訊（如Novell、Banyan、IP等）。網橋的缺點是它只能連接兩個同類型的網絡，連接物理上不相鄰的網絡十分困難。</p><p

22、>　　路由器可連接更多的IP（即Internet Protocol）網絡。網絡上的主機必須意識到路由器的參與，但這對IP網絡來說不成問題，因為IP的規(guī)則之一便是所有的主機必須能與路由器對話。</p><p>　　路由器的優(yōu)點之一是能連接物理不同的網絡，如將一快速的局Ethernet網連于低速的長途電話線。路由器的缺點是傳遞包的速度不及網橋，因為它需更多的計算來決定如何傳送包。</p><

23、p>　　網關可拼接兩種不同的協(xié)議．例如，你的網絡使用IP，他人的網絡采用Novell、DECnet、SNA或其它名牌網絡的某種協(xié)議，網關就將通訊從一種協(xié)議轉化成另一種。網關不僅有特殊的協(xié)議，還有特定的應用程序。記住，網橋（bridges)、路由器（routers）和網關(gateway）三者的區(qū)別是由軟件而定的，所以某些情況下可根據使用的軟件，將同樣的硬件裝配成網橋、路由器或網關是完全可能的。</p><p&g

24、t;　　在分時系統(tǒng)中，用戶通過終端訪問計算機，有些是通過電纜線直接和計算機相聯(lián)的本地終端，而另外一些是遠程終端，通過電話線或其他傳輸介質與遠距離的計算機通信。遠程計算涉及許多特殊的問題，我們就來研究一下這些問題。 </p><p>　　在計算機內部，數據表示為一些離散的電脈沖——0和l。由于系統(tǒng)的各部件一般相互之間在幾尺的范圍內，因此這些數位能很容易地從一個部件傳送到另一個部件。然而，當我們要遠距離地發(fā)送電

25、脈沖信號時就會出現一些問題。首先，由于導線的阻抗使信號失去了強度，或者說逐漸減小。與此同時，信號還要接收干擾或噪音，收聽遠距離無線電臺時，電子信號外來雜聲中的靜態(tài)干擾就是一個很好的例子。當信號從它的發(fā)射源向遠處傳送時，信號變得越來越弱，噪聲則變得比較強，直到最后信號被淹沒了。如果數據要遠距離發(fā)送，噪聲就必須被濾掉，信號還要不時放大。</p><p>　　數據傳輸通常采用載波信號，如正弦波。一個完整的的正弦波形稱為

26、一個周期。從波形的最低點至最高點的高度是它的幅度，每秒鐘周期的數量是波的頻率。因為載波信號是用己知的頻率和幅度傳輸的，所以就可以設計一種設備去對載波信號濾波和放大。</p><p>　　載波的這些性質如何才能用來編碼和傳輸二進制數據呢?很簡單。首先，要用給定了幅度和頻率的標準波形，讓每個周期表示一個信號位。對于傳輸一個1數位．保持波形不變；對于傳輸一個0數位，則改變一個周期的頻率。換言之，一個正常的周期表示一個1

27、，而其他所有情況表示一個0。這樣就得到了一個表示數位序列的連續(xù)信號。數據是離散的電脈沖，波是表示數據的一個模擬量。我們每天都用模擬量，溫度計中的水銀柱的高度不是實際的溫度．它代表溫度：汽車控制儀表板上的指針位置不是速度，它代表速度；通過一個通信線路的連續(xù)波不是數據，但它模擬了數據。</p><p>　　然而，計算機并不能以連續(xù)波的形式存儲數據，計算機存儲和處理離散脈沖。由于這種電子信號的不兼容性．無論何時當數據在

28、計算機和遠程終端之間傳輸時，它們必須從脈沖形式轉換成連續(xù)波形式，并且還要轉換回來。把離散的脈沖轉換成連續(xù)波的形式稱調制，再轉換回來稱解調。這項任務由稱為數傳機或調制解調器的硬設備來完成。通常，在通信線路的每一端部有一個這種設備。</p><p>　　因為直到現在我們才能經濟地組裝出一些設備去處理數字信號，所以許多通信線路還是傳輸模擬信號。電子學方面的進展不僅已經使數字數據傳輸成為可能，而且變得很普遍。事實上，大量

29、的新的通信設備是數字式的．用數字通信不存在連續(xù)的載波，而是把數值作為簡單的離散脈沖傳輸。這些數字信號受噪音的干擾?。虼耍瑪祿芤员容^高的精度傳輸。</p><p>　　大概電話網絡是人所共知的數據通信介質，一般的音頻線路傳輸速率約為2400 b/s或2400波特。高速的寬頻帶通道能以接近1百萬波特的速率傳輸。在這兩個極限值之間的幾種波特率都是可用的。微波數據傳輸可代替電話線路，不幸的是微波傳輸距離受視野范圍的限

30、制。眾所周知，地球是圓的，它的表面是彎曲的，這個曲率限制了微波的傳輸范圍，這就需要建立一些費用昂貴的中繼站相通信衛(wèi)星。</p><p>　　我們可花大量的時間討論各種通信介質，但那會造成不必要的混淆。我們采用線路這個通用的術語來描述任意一種數據通信介質。</p><p>　　假設若干終端都鏈接到一個中央計算機上．在計算機端按每秒鐘近幾百萬個字符的速度處理數據，常用的2400波特的線路大約傳

31、輸300字符/秒，終端情況又如何呢?已知大多數人錄入的速度，也許每秒鐘從鍵盤鍵入10個字符說起來已是很夸張了。我們把一個每秒鐘可鍵入10個字符的鍵盤接到每秒鐘可傳輸300個字符的線路上，同樣，這個線路再接到每秒鐘能處理幾百萬個字符的計算機上，那是多大的一個差距啊！</p><p>　　足以保存一個事務或一滿屏幕數據的緩沖區(qū)能幫助終端和它的通信線路同步。當用戶鍵入時，字符被存入在緩沖區(qū)內。錄入完成后．用戶按下回車鍵

32、。響應這個信號，緩沖區(qū)的內容就按線路的傳輸速率傳送。在輸出端，數據通過線路傳送．并進入緩沖區(qū)。緩沖區(qū)里的數據按終端的速率被顯示或打印。用一個居中的緩沖區(qū)就能使終端和線路之間的速度相匹配了。</p><p>　　在線路的另一端又發(fā)生了什么情況呢?一個現代的傳輸控制單元常稱為前端設備，是由一組端口和相關的緩沖區(qū)組成的。端口是與通信線路連接的點，一般包含調制和解調所需要的電子設備。數據按通信線路所確定的速率進入傳輸控制

33、單元。并進入到與“輸入端口”相關的緩沖區(qū)內?！┧脭祿M入緩沖區(qū)后，傳輸控制單元就發(fā)一個信號給通道，接著，通道發(fā)一個信號給計算機準備輸入數據。計算機一準備好，數據就按計算機的內部處理速度通過總線線路傳輸。在輸出端，數據按計算機的速度傳送給控制單元，然后，以比計算機低得多的速度把打好包的數據發(fā)送給通信線路。</p><p><b>　　附錄4</b></p><p>

34、<b>　　Internet</b></p><p>　　The Internet also known as the Net is the world's largest computer network. “ And what is a network”? You may ask. A computer network is basically a bunch of compu

35、ters booked together somehow. In concept, it's sort of like a radio or TV network that connects a bunch of radio or TV stations.</p><p>　　Some computer networks consist of a central computer and a bunch

36、of remote stations that report to it-a central airline reservation computer, for example, with thousands of terminals at airports and travel agencies. Other, including the Internet, permits any computer on the network to

37、 communicate with any other.</p><p>　　The Internet is probably the most open network in the world. Thousands of computers provide facilities that are available to anyone who has Net access. This situation is

38、 unusual-most networks are very restrictive in what they allow users to do and require a specific arrangements and passwords for each service. Although a few pay services exist, the vast majority of Internet services are

39、 free for the taking, once you connected. If you don't already have access to the Internet through your company</p><p>　　“How do I get to the Internet?” Well depending on your lifestyle, including your j

40、ob or your school, checking out the Internet can be trivially easy or moderately difficult.</p><p><b>　　(1) Modem</b></p><p>　　A modem is a little device that enables data from one c

41、omputer to travel to another computer by using ordinary telephone lines. This cute little box goes between your and your phone line and between the phone line and the computer on the other end. Modem are built-in to some

42、 computer, so if you've not sure whether one is in your computer, ask.</p><p>　　If you need to buy a modem, find the cheapest fax modem that runs at 14400hps. You can pay more for things that go faster,

43、but that speed isn't really passed on to you by using your ordinary phone line just now. Make sure that your modem comes with some communications software that's appropriate for the computer and version of operat

44、ing system you're using.</p><p>　　The Internet is out there with tons of exciting things to do, but you have to it through your school or work. The folks who provide access to the Internet (generally for

45、 a fee) are called Internet Service Providers.</p><p>　　(2) On-line</p><p>　　On-line service is services provided to you by a computer or computers interactively. They include but are not limite

46、d to computerized banking, shopping, dating, entertainment, and study. They may or may not have anything to do with the Internet, such as CompuServe, existed independently of the Internet. They allowed users to exchange

47、electronic mail among other CompuServe users but failed to reach out to touch anyone else. Now most of these services do provide e-mail access beyond their insulat</p><p>　　(3) Bulletin boards (BBS)</p>

48、;<p>　　Electronic bulletin board systems (abbreviated as BBS) provide on-line services generally on a smaller scale. Bulletin board system is often local, often very cheap, and sometimes even free to callers. They

49、 might provide e-mail, cheating (on-line conversations with other users), and forums about special interests, games, ads, and lots, lots more.</p><p><b>　　Gateway</b></p><p>　　The fi

50、rst question is whether your PC has a native connection to the Internet or whether it goes through some other type of network. If it has a native connection, your PC runs network software that handles the TCP/IP network

51、protocols the Internet uses.</p><p>　　The alternative is that your PC is running another kind of network software (most often Novell’s Netware) and is attached to a gateway system that speaks Netware on one

52、side and TCP/IP on the other. It’s also possible to load up both TCP/IP and some other kind of network software on the same PC and run them on the same physical network cables at the same time.</p><p>　　If y

53、ou already have a large Network, your system manager can load up a single Netware gateway that all the other PCs on the network can see. If each PC is to run TCP/IP itself, your network manager has to load TCP/IP on each

54、 PC, which is a great deal more work.</p><p>　　TCP/IP works better in heterogeneous networks-that is, networks made up of lots of different kinds of computers. Novell Netware is great on networks of PCs and

55、less great on other kinds of computers because Netware at this point works on far fewer kinds of computers than TCP/IP does.</p><p>　　Ethernet Token</p><p>　　Assume that your PC runs TCP/IP by i

56、tself and not by way of anther kind of network. How is it connecting to the rest of the Net? The three major choices are Ethernet, Token Ring, and serial lines. The most common way to hook up PCs to networks is with Ethe

57、rnet, a kind of network that is fast and cheap but limited to a total distance of less than a mile. (For larger network, a person can hook together several Ethernet network by using devices called bridges and routers.<

58、;/p><p>　　Token Ring </p><p>　　A few years back, IBM decided that because every other computer manufacturer in the world was using Ethernet, it would avoid the heartache of excessive compatibility

59、by coming up with its own network called Token Ring. Furious technical debates raged on about the relative merits of Ethernet and Token Ring, which we can summarize by saying that the differences are 95 percent political

60、. Token Ring uses a clever rectangular connector that has the advantage over many version of Ethernet in that it c</p><p>　　The Internet has more than a million computers attached to it. How do you find the

61、one you want? A number and a name identify each machine on the net.</p><p>　　Each machine on the net (call a host) has a number assigned to identify it to other hosts, sort of like a phone number. The number

62、s are in four parts, such as 123.45.67.89.</p><p>　　Most hosts also have names, which are much easier to remember than numbers are. The names have multiple parts separated by dots (chico.iecc.com, for exampl

63、e). Some hosts have more than one name, but it doesn’t matter which of them you use. Each network in the Internet has rules about what kinds of network traffic (e-mail, terminal sessions, and other connections) it allows

64、. You should know the rules that apply to the network (or networks) you use to avoid getting the network managers mail at you</p><p>　　Any computer of any kind, from the smallest to the largest, attached to

65、the Internet is called a host. Some hosts are giant mainframes or supercomputers that provide services to thousands of users; some are little workstations or PCs with one user; and some are specialized computers, like ro

66、uters, which connect one network to another, or terminal servers. But from the Internet’s point of view, they’re all hosts.</p><p>　　Each machine is assigned a host number, which is sort of like a phone numb

67、er. Being computers, the kind of numbers hosts like is 32-bit binary numbers. For example,</p><p>　　10001100101110100101000100000001</p><p>　　That’s not very memorable. To make the number slight

68、ly easier to remember, it’s broken up into four 8-bit groups, and then each group is translated into a decimal equivalent. So our number turns into</p><p>　　140.186.81.1</p><p>　　Internet host n

69、umbers are also divided into two parts: The first part is the network number, and the second part, the local part, is a host number on that particular network. In the case of our computer, 140.186.81.1 means network numb

70、er 140.186, and local host number (on that network) 81.1.</p><p>　　Because some networks have many more hosts on them than other do, networks are divided into three sizes. In large networks (Class A), the fi

71、rst of the four numbers is the network number, and the last three are the local part. In medium networks (Class B), the first two numbers are the network number, and the last two are the local part. In small networks (Cl

72、ass C), the first three numbers are the network number, and the last is the local part.</p><p>　　The first of the four numbers tells you in which class the network is. Frequently an organization that has a s

73、ingle network number wants to set up its computers internally on multiple networks. All the computers in a single department, for example, are usually attached together on a single network, with some sort of connection l

74、inking together department networks. But adhering to the way the Internet was originally set up would mean that an outfit with 25 internal networks would have to get 25 d</p><p>　　It meant that every time a

75、company set up a new internal network, it had to apply for a new network number. The rest of the Internet world had to put that network number in their tables so that they knew how to route messages to it.</p><

76、;p>　　Clearly, something had to be done. That something is called a subnet. All that means is that one network can be divided into pieces called subnets. On a subnet, part of what would normally be the host number beco

77、mes part of the network number. In network 140.186, for example, the third number in the host number is the subnet number, so for machine 140.186.81.1, the subnet number is 140.186.81, and the host number is 1. This enab

78、les plenty of local networks to be installed (we use only 90 of the </p><p>　　In practice, all but the smallest networks are subnet Ted.</p><p>　　A few organizations have an opposite problem. Th

79、ey have too many computers for a Class C network, more than 254, but nowhere near enough to justify a Class B. In this case, the organization can get a block of adjacent network numbers and treat part of the network numb

80、er as a host number, a process called super netting.</p><p>　　Normal people use names, not numbers, so Internet host and usually referred to by name, not by number. To avoid a crisis of naming creativity, th

81、e solution was to go to multipart names; a scheme grandly known as the Domain Name system or DNS. Host names are a string of words separated by dots.</p><p>　　The rightmost part of a name is called its zone.

82、 The three letter zones are set up by type of organization such as following Table:</p><p>　　Two-letter zone names are organized geographically. Each zone corresponds to a country or Other Political Entity.

83、For example, the country for Canada is CA, so a site at York University in Canada is called nexus, yoke, ca. If you have one or two machines, it’s easier to get a geographic name. If you have more than that, it’s easier

84、to get an organizational name, which lets you administer names within your organization yourself.</p><p>　　What the Internet does, basically, is transmit data from one computer to anther. If you want to mail

85、 a package to someone, you write the recipient’s address and your return address on it and mail it. The package is routed from your local post office to a central post office, where it’s then loaded on a truck or a train

86、 headed in the right general direction and passed repeatedly from office to office until it gets to the recipient’s post office, at which point the letter carrier delivers it to yo</p><p>　　The Internet work

87、 is much closer to how the post office does. Each time a host wants to send a message to another host, the sender sends the message to a host that can forward it. It’s quite common for a pass through a dozen or more forw

88、arders on its way from one part the net to another.</p><p>　　The main complaints people have about the post office are that it’s slow and that it loses stuff. The Internet occasionally has both of these prob

89、lems. In the middle of a busy day, the net can indeed slow down, although the time a message takes to be delivered is still measured in seconds.</p><p>　　The various networks on the Internet work in pretty m

90、uch the same way; They pass data around in chunks called packets, each of which carries the addresses of its sender and its receiver (those host number we talked about earlier). The maximum size of a packet varies from n

91、etwork to network, but it is usually between 200 and 2000 octets.</p><p>　　The set of conventions used to pass packets from one host to another is known as the Internet Protocol, or IP.</p><p>　

92、　The two best known are Transmission Control Protocol (TCP) and User Data gram Protocol (UDP). TCP is so widely used that many people refer to TCP/IP, the combination of TCP and IP used by most Internet applications.<

93、/p><p>　　Three kinds of things pass packets from one network to another: bridges, routers, and gateways.</p><p>　　A bridge connects two Networks in a way that makes them appear to be a larger netwo

94、rk. Bridges are used most commonly to connect two Ethernet local area networks. The bridge looks at all the packets flying by on each of the network is to build a large table listing which hosts are on each network, base

95、d on the return addresses on all the packets flying by on each network.</p><p>　　A router connects two or more IP (that’s the Internet Protocol) network. The hosts on networks have to be aware that a router

96、is involved, but that’s no problem for IP networks because one of the rules of is that all host have to be able to talk to routers.</p><p>　　A gateway splices together two different kinds of protocol. If you

97、r network talks IP, for example, and someone else’s network talks Novell or DECnet or SNA or one of the other dozen Leading Brands of Network, a gateway converts traffic from one set of protocols to another. Gateways are

98、 not only specific to particular protocols but also are application-specific.</p><p>　　On a time-shared system, users access a computer through terminals. Some terminals are local, lined directly to a comput

99、er by cables, while others are remote, communicating with distant computers over telephone lines or other transmission media. Remote computing involves a number of special problems. Let’s investigate.</p><p>

100、;　　Within a computer, data are represented as discrete electrical pulses-0s and 1s. Since the system’s components are normally within a few feet of each other, these bits can easily bi moved from component to component.

101、However, when we attempt to transmit electrical pulses over a distance, several things happen. First, the signal loses intensity or “dies down” because of the resistance of the wire. At the same time, it picks up interfe