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`APPLE 1005
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`Illustrated by MICHAEL TROLLER
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`APPLE 1005
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`How the
`Internet
`Works
`Sixth Edition
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`How the
`internet
`Works
`
`Sixth Edition
`
`Preston Gralla
`
`Ou’
`
`201 W. 103rd Street
`Indianapolis, IN 46290
`
`5
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`6
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`

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`How the Internet Works, Sixth Edition
`Copyright © 2002 by Que
`
`Associate Publisher
`Acquisitions Editor
`Development Editor
`Managing Editor
`Project Editor
`Production Editors
`
`Indexer
`
`Technical Editor
`
`Illustrator
`
`Team Coordinator
`Page Layout
`
`Greg Wiegand
`Stephanie J. McComb
`Nicholas J. Goetz
`ThomasF. Hayes
`Karen S. Shields
`Megan Wade
`Amy Jay
`Erika Millen
`
`Chris Faust
`
`Michael Troller
`
`Sharry Lee Gregory
`Gloria Schurick
`
`All rights reserved. No part of this book shall be reproduced, stored in a retrieval system, or transmitted by any means,
`electronic, mechanical, photocopying, recording, or otherwise, without written permission from the publisher. No patent
`liability is assumed with respect to the use of the information contained herein. Although every precaution has been takenin
`the preparation of this book, the publisher and author assume no responsibility for errors or omissions, Noris anyliability
`assumed for damages resulting from the use ofthe information contained herein.
`International Standard Book Number: 0-7897-2582-7
`
`Library of Congress Catalog Card Number: 20-01087727
`Printed in the United States of America
`
`First Printing: September, 200!
`04
`03
`02 Ol
`43 24
`
`Trademarks
`All terms mentioned in this book that are known to be trademarks or service marks have been appropriately capitalized. Que
`cannotattest to the accuracy of this information. Use of a term in this book should not be regarded as affecting the validity of
`any trademark or service mark.
`Warning and Disclaimer
`Every effort has been made to make this book as complete and as accurate as possible, but no warranty or fitness is implied.
`Theinformation provided is on an “asis” basis. The author and the publisher shall have neitherliability nor responsibility to any
`person or entity with respect to any loss or damages arising from the information contained in this book.
`
`7
`
`

`

`
`
`Introduction
`
`Part |: Understanding the
`Internet’s Underlying
`Architecture
`|
`
`Chapter 1
`WhatIs the Internet?
`Chapter 2
`How Computer Networks Send Data
`Across the Internet
`
`Chapter3
`How TCP/IP Works
`Chapter 4
`Understanding the Internet’s Software
`Structure
`
`Chapter 5
`How Internet Addresses and
`Domains Work
`
`Chapter 6
`How Routers Work
`
`Chapter 7
`How Internet File Types Work
`
`4
`
`8
`
`12
`
`16
`
`20
`
`28
`
`32
`
`Chapter 13
`How You Connectto the Internet from
`Online Services
`Chapter 14
`Ilow Wireless and Palmtop Connections
`Work
`
`Chapter 15
`How Home Networks Work
`
`64
`
`68
`
`78
`
`Part 3: Communicating on
`the Internet
`
`84
`
`Chapter 16
`How E-mail Works
`
`Chapter 17
`How E-mail Spam Works
`Chapter 18
`How Newsgroups Work
`Chapter 18
`HowInternet Chat and Instant
`Messaging Work
`Chapter 20
`HowInternet Phone Calls Work
`
`88
`
`100
`
`106
`
`110
`
`116
`
`Part 2: Connecting to the
`Internet
`
`36
`
`Part 4: Using the World
`Wide Web
`
`122
`
`Chapter8
`How Computers Connect to
`the Internet
`Chapter 9
`Howa Modem Works
`Chapter 10
`How Internet/Television Connections
`Work
`Chapter 11
`How the Digital Subscriber Line (DSL)
`Works
`
`Chapter 12
`HowISDN Works
`
`40
`
`44
`
`48
`
`56
`
`60
`
`Chapter 21
`How Web Pages Work
`Chapter 22
`How Web Browsers Work
`Chapter 23
`How Markup Languages Work
`Chapter 24
`How Hypertext Works
`Chapter 25
`How URLs Work
`
`Chapter 26
`How Image MapsandInteractive
`Forms Work
`
`126
`
`134
`
`140
`
`148
`
`152
`
`156
`
`8
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`Chapter 27
`How Web Host Servers Work
`
`Chapter 28
`How Web Sites Work with Databases
`Chapter 29
`HowInternet-Based Software Works
`
`160
`
`164
`
`168
`
`Part 5: Using Common
`Internet Tools
`
`172
`
`Chapter 30
`How Tenet Works
`
`Chapter 31
`How FTP Downloading Works
`Chapter 32
`HowInternet Searching Works
`Chapter 33
`How Agents Work
`Chapter 34
`How Java, ActiveX, and JavaScript
`Work
`
`Chapter 35
`How CGIScripting Works
`
`176
`
`180
`
`186
`
`192
`
`196
`
`204
`
`Part 6: Enjoying
`Entertainment and
`Multimedia on the
`Internet
`208
`
`Chapter 36
`How Music and Audio Work on
`the Internet
`
`Chapter37
`How Napster and File Sharing Work
`Chapter 38
`How Video onthe Internet Works
`Chapter 39
`How Multicast IP and the MBone
`Work
`
`212
`
`220
`
`224
`
`232
`
`Chapter 40
`How Virtual Reality Works
`Chapter 41
`Animation on the Web
`
`236
`
`240
`
`Part 7: Shopping and Doing
`Business on the
`Internet
`
`248
`
`Chapter 42
`How Intranets Work
`
`Chapter 43
`Shopping on the Internet
`
`252
`
`258
`
`Part 8: Protecting Yourself
`onthe Internet
`268
`
`Chapter 44
`How Firewalls Work
`Chapter 45
`How Hackers Can Cripple the Internet
`and Attack Your PC
`
`Chapter 46
`How Viruses Work
`
`Chapter 47
`How Internet Sites Can Invade
`Your Privacy
`Chapter 48
`Cryptography, Privacy, and Digital
`Certificates
`
`Chapter 49
`Howthe FBI’s “Carnivore” Program
`Works
`
`Chapter 50
`Parental Controls on the Internet
`Glossary
`Index
`
`272
`
`280
`
`286
`
`292
`
`302
`
`308
`
`312
`316
`326
`
`9
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`

`

`Acknowledgments
`|} 1Sbook,like the Internet, is a collaborative work. My name might be on the cover, but I am far
`from the only person involved in its creation.
`Development editor Nick Goetz managedto pull together all the elements of a complex project,all
`the while keeping a cool head, improvingtheillustrations, and offering cditorial insight and lelp. Aud
`acquisitions editor Stephanie McCombwas instrumental in getting the book off the ground, andas al-
`ways, was a pleasure to work with.
`Theillustrator for the book, Michael Troller, deserves much thanks. Not only did he create newil-
`lustrations from scratchspecifically for this book, he redid all the ones from past books and madethis
`edition a more coherent, richer, more visually pleasing experience.
`Thanksalso go to illustrators of past editions: Mina Reimer, Sarah Ishida, Shelly Norris, and Stephen
`Adams. And manythanksto the entire team at Que who produced the book, including Karen Shields,
`Megan Wade, AmyJay, Gloria Schurick, and Erika Millen.
`Thanksalso have to go to the many, many people | interviewed for this book. People from
`Quarterdeck Corporation, Chaco Communications, Progressive Networks, White Pine Software,
`Microsoft, Netscape, Headspace, SurfWatch Software, WebTV, Accrue, VDONet Corporation, America
`Online, Yahoo!, Hilgraeve, Fairmarket, eWallet, and Nuborn Technologies are only a few of the folks
`who gave their time to help me understand thenitty-gritty of how various Internet technologies work.
`Tim Smith from ZDNetoffered me vital help as well.
`I gleaned much information from the many FAQsand similar documents widely available on the
`Internet.I’d like to thank the anonymousauthors of those documents, whoevertheyare.
`Many thanks go to Chris Faust, the technical editor for the book, who did a superb job of ensuring
`that I always got the information right.
`Finally, big thanks have to go to my wife, Lydia. She put up with those occasional glassy-eyed looks
`that were replies to simple questions like, “Did you leave your keys in the refrigerator again?” Shealso
`endured my extreme absent-mindedness while | was figuring out ways to explain how firewalls, ISDN, or
`Web robots work when I should have been concentrating on more immediate matters.
`
`10
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`10
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`

`

`aSSSere
`
`Tell Us What You Think!
`AS the readerof this book, you are our most importantcritic and commentator. We value your opinion
`and want to know what we’re doing right, what we could do better, what areas you’d like to see us pub-
`lish in, and any other words of wisdom you're willing to pass our way.
`As an associate publisher for Que, I welcome your comments. You can fax, e-mail, or write me di-
`rectly to let me know whatyoudid or didn’t like about this book—as well as what we can do to make
`our books stronger.
`Please note that I cannot help you with technical problemsrelated to the topic of this book, and
`that due to the high volumeof mail I receive, I might not be able to reply to every message.
`Whenyou write, please be sure to include this book’s title and authoras well as your name and
`phoneor fax number.I will carefully review your comments and share them with the author and editors
`who worked on the book.
`
`Fax:
`E-mail:
`Mail:
`
`317-581-4666
`feedback@quepublishing.com
`Greg Wiegand
`Associate Publisher
`Que
`201 West 103rd Street
`Indianapolis, IN 46290 USA
`
`11
`
`11
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`

`

`Introduction
`1. the course of cruising the World Wide Web andclicking a link, have you ever wondered,
`“How does that work?” Or perhaps this question popped into your mind while you were trans-
`ferring a file to your computer via FTP, or reading a newsgroup message, or when youfirst
`heard about technologies such as spam, cookies, and firewalls. Maybe you've wondered how
`a message sent from your computer travels through the vastness of cyberspace and ends up
`in the right e-mail box halfway across the world. Have you ever wanted to know howsearch
`tools find the exact piece of information you wantoutof the millions of pieces of information
`on the whole Internet? How canyoulisten to music and view animations while surfing the
`Web?
`
`This book is designed for everyone interested in the Internet. Its guiding principle is this:
`No matter how muchof a cyberpro you are—or how muchof a novice—there’s a lot you
`don’t understand aboutthe Internet. Here’s just one small example. I have a friend who has
`madehis living with companies involved with the Internet for many years. He’s a complete
`cyberpro wholives and breathes the Internet. One day, he almost whispered to me,“I don’t
`yy
`like to admit this, but ] don’t know whata proxy serveris. How does it work, anyway?
`
`He’s not alone. The Internet changes so quickly and the technology advancesso rapidly
`that it can seem almost impossible to keep up withallofit. If you're like just about everyone
`else involved in the Internet, your questionsare similar to those of my friend. You'll find your
`answers here.
`
`In Part 1, “Understanding the Internet’s Underlying Architecture,” I explain the underly-
`ing basics of the Internet: who runsit, how TCP/IP works, how to understand Internet
`addresses and domains, and similar topics, Here’s where you'll find out about things such as
`routers and how the client/server architecture underpins virtually every aspect of the Internet.
`
`Part 2, “Connecting to the Internet,” depicts the various ways you can connect your com-
`puter to the Internet. Here’s where toturnif you’re interested in any of the following topics:
`how a cable modem, a digital subscriber line (DSL), or ISDN works; how online services con-
`nect to the Internet; how you can connectby satellite to the Internet; how “palmtop” com-
`puters, such as 3COM’s Palm, can browse the Web; how a modem works; how you can access
`the Internet from your own home network; and a host of similar subjects.
`
`Part 3, “Communicating on the Internet,” covers every aspect of Internet communica-
`tions. It shows how e-mail and newsgroups work, how IRC chat works, what e-mail “spam” is
`and what you can do to preventit, how instant messaging works, and how you can use the
`Internet to make telephone calls anywhere in the world.
`
`12
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`12
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`

`

`Part 4, “Using the World Wide Web,” covers what has becomeby far the most popular
`part of the Internet—the World Wide Web. You'll learn virtually every aspect of how the
`Web works. It delves into how browsers work, how Web server software works, and how
`Hypertext Markup Language (HTML) works. This section also covers the ways in which the
`Web is becoming integrated directly into your computer, how Web pages are published and
`organized onasite, and every other aspect of the Webthatis likely to be of interest to you.
`
`Part 5, “Using Common Internet Tools,” takes a close look at common Internettools.
`Here’s the place to learn aboutbasic tools and services, such as telnet, and what happens
`when you use FTP to downloadafile to your computer. You'll also learn how search engines
`and search tools work. Andit’s the section of the book that covers cutting-edge Internet
`technology. You'll find out about how push technology works, how the Java and ActiveX pro-
`gramming languages work, how JavaScript works, how agentscansilently do your biddingfor
`you, and how the Common GatewayInterface (CGI) works, whichis a little-known butvital
`part of the Internet.
`
`Part 6, “Enjoying Entertainment and Multimedia on the Internet,” shows you how some
`of the most exciting parts of the Internet work—thevarious multimedia technologies.
`Whether you want to know howvirtual reality or animations work, how streaming video
`works, how videoconferencing works, or how similar technologies work, you'll find it all here.
`
`Part 7, “Shopping and Doing Business on the Internet,” covers intranets, how the
`Internet works with the outside world, and how you can shop online. You'll see how compa-
`nies use Internet technologies to build their own private networks, called intranets. And you'll
`take a close look at the underlying technologies that let you shop on the Web, which
`accounts for billions of dollars a yearin sales.
`Finally, Part 8, “Protecting Yourself on the Internet,” covers security concerns. It explains
`the controversial cookie technology that lets Web servers put bits of information on your
`hard disk and use that information to track you. It explains how the even-more controversial
`FBI system called Carnivore can in essence wiretap people’s use of the Internet to read their
`e-mail, watch their Web browsing, and more. This part also shows how firewalls work, how
`viruses can attack your computer, and how cryptosystemsallow confidential information to
`be sent across the Internet. It delves into how hackers cai attack Internet service providers
`(ISPs) using so-called “smurf attacks,” and how they can attack your computeras well. Andit
`covers the issue of pornography on the Internet and shows how parental-controlsoftware can
`prevent children from seeing objectionable material.
`
`So, come along and see how the vast Internet works. Even if you're a cyberpro (and espe-
`cially if you’re not), you'll find out a lot you never knew,
`
`13
`
`13
`
`

`

`PART | UNDERSTANDING THE INTERNET’S UNDERLYING ARCHITECTURE
`
`
`
`14
`
`14
`
`

`

`UNDERSTANDING
`THE INTERNET’S
`UNDERLYING
`ARCHITECTURE
`
`Chapter 1: WhatIs the Internet?
`+
`
`Chapter 2: How Computer Networks Send Data Across
`the Internet
`8
`
`Chapter 3: How TCP/IP Works
`12
`
`Chapter 4: Understanding the Internet’s Software Structure
`16
`
`Chapter 5: How Internet Addresses and Domains Work
`20
`
`Chapter 6: How Routers Work
`28
`
`Chapter 7: How Internet File Types Work
`32
`
`15
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`

`

`
`
`POR thefirst time ever, the worldis at yourfingertips, From your computer, you can find
`information about anything you can nameor even imagine. You can communicate with
`people on the otherside of the world. You can set up a teleconference, tap into the resources
`of powerful computers anywhere on the globe, search through the world’s best libraries, and
`visit the world’s most amazing museums. You can watch videos, listen to music, and read spe-
`cial multimedia magazines. You can shop for almost anything you can name. You can do all
`this by tapping into the largest computer network in the world—theInternet.
`
`The Internetisn’t a single network;it is a vast, globe-spanning network of networks. No
`single person, group, or organization runs the Internet. Instead,it’s the purest formof elec-
`tronic democracy. The networks communicate with one anotherbased on certain protocols,
`such as the Transmission Control Protocol (TCP) and the Internet Protocol (IP). More and
`more networks and computers are being hooked up to the Internet every day. Tens of thou-
`sands of these networks exist, ranging from university networks to corporate local area net-
`worksto large online services such as America Online and CompuServe. Every time you tap
`into the Internet, your own computer becomes an extension of that network.
`
`Thefirst section of this book is spent defining the Internet. It also examines the architec-
`tures, protocols, and general concepts that makeitall possible.
`
`Chapter 1, “WhatIs the Internet?,” examines how the Internet runs. You’ll look at who
`pays for the high-speed data backbones that carry much of the Internet’s traffic and at the
`organizations that ensure that standardsare set for networks to follow so the Internet can run
`smoothly, You'll also look at the various types of networks connected to the Internet.
`
`Chapter 2, “How Computer Networks Send Data Across the Internet,” explains how
`informationtravels across the Internet and describes how hardware such as routers, repeaters,
`and bridges sends information among networks. It also shows howsmaller networks are
`grouped into larger regional networks—and how those large regional networks communicate
`among themselves.
`
`Chapter3, “How TCP/IP Works,” covers the Internet’s basic protocols for communica-
`tions. You'll learnalittle about basic Internet jargon, such as TCP/IP (short for Transmission
`Control Protocol and Internet Protocol), This chapter also explains how those protocols work
`and how special software such as Winsock enables personal computers to get onto a network
`originally designed for larger computers.
`
`Chapter 4, “Understanding the Internet’s Software Structure,” looks at the Internet's
`client/server architecture. Servers—also called hosts—are powerful computers that perform
`functions such as delivering information or Web pages, hosting databases, and handling
`e-mail. A client is your own computer and the software thatsits on it, such as a Web browser
`
`16
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`16
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`

`

`or piece of e-mail software. Clients request information from servers, which do the heavy-
`duty processing and then send the information back to the client, which displays the infor-
`mation.
`
`Chapter 5, “How Internet Addresses and Domains Work,” takes the mystery out of the
`Internet’s often confusing addressing scheme, You'll learn about Internet domains and
`addresses and will even be able to make sense of them. More than that, you'll learn how
`domainservers are capable of keeping track of all the locations on the Internet andtranslat-
`ing addresses such as www. zdnet.com into Internet IP addresses such as 134.54.56. 120.
`You'll also learn how some computersare assigned new IP addresses by special servers every
`time they connectto the Internet.
`
`Chapter 6, “How Routers Work,” details how the most basic piece of hardware on the
`Internet—a router—works. Routers are combinations of hardware and software that perform
`the job of ensuring thatall data is sent to the properdestination. Think of routers as traffic
`cops of the Internet. They use the IP addresses the nameservers havetranslated to route the
`data. Routers look at the addresses and then send the data to the next-closest router to the
`destination and so on,until the datais finally delivered. They use routing tables to determine
`how to route the traffic, and they can also adjust the routesas the traffic on the Internet
`changes,thus ensuring that the data is routed in the mostefficient way possible.
`Chapter 7, “How Internet File Types Work,” gives you an understanding of the most com-
`montypes offiles you’ll come across when you browse the Net. Compressedfiles, videofiles,
`graphicsfiles—you'll learn about almost every typeoffile you might encounter.
`Whether you’re a newbie or cyberpro, this section teaches you the basics of the Internet.
`
`17
`
`17
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`

`

`CHAPTER What Is the
`i
`Internet?
`
`
`
`18
`
`

`

`ONE of the most frequently asked questions about the Internet is: Who runsit? The truth is that no cen-
`tralized managementofthe Internet exists. Instead, it is a collection of thousands of individual networks
`and organizations, each of which is run and paid for on its own. Each network cooperates with other net-
`worksto direct Internet traffic so that information can pass among them. Together, these networks and orga-
`nizations make up the wired world of the Internet. For networks and computers to cooperate in this way,
`however, a general agreement must take place about things such as Internet procedures and standards for
`protocols. These procedures and standardsare laid out in requests for comment (RFCs) that are agreed on by
`Internet users and organizations.
`A variety of groups guide the Internet’s growth by helping to establish standards and by educating people
`on the proper way to use the Internet. Perhaps the most importantis the Internet Society, a private, non-
`profit group. The Internet Society supports the work of the Internet Activities Board (LAB), which handles
`much of the Internet’s behind-the-scenes and architectural issues. The Internet Engineering Task Force
`(IETF)is responsible for overseeing how the Internet’s TCP/IP protocols evolve. For information about the
`IETE, go to www. ietf.org.(See Chapter 3, “How TCP/IP Works,” for details on TCP/IP protocols.)
`The World Wide Web Consortium (W3C) develops standards for the evolution ofthe fastest-growing
`part of the Internet, the World Wide Web(find it at ww.w3.org). The W3Cis an industry consortium run
`by the Laboratory for Computer Science at the Massachusetts Institute of Technology (MIT).
`Private companies oversee the registering of Internet domains, such as www. zdnet. com or
`Www. quepublishing.com. These companies, called registrars, all must cooperate with one another to
`ensure that only one person or company can owna particular domain andthat all the domains work prop-
`erly. The registrars compete with one anotheras well in allowing people and businesses to register domains.
`Registering a domain costs money, and the registrars compete on cost and on giving extra services to those
`who buy domains.
`Althoughall these types of organizations are important for holding together the Internet, at the heart of
`the Internet are individual local networks. These networks can be found in private companies, universities,
`governmentagencies, and online services. They are funded separately from each other and in a variety of
`manners, such as fees from users, corporate support, taxes, and grants. Many Internet service providers
`(ISPs), which provide Internet access for individuals, have networks as well. Individuals who wantto access
`the Internet pay ISPs a monthly connection rate, so in that sense, everyone who uses the Internet helps pay
`for it.
`
`The networks are connected in a variety of ways. For efficiency’s sake, local networks join in consortiums
`knownas regional networks. A varlety of leased lines connect regional and local networks. The lcased lines
`that connect networks can be as simple as a single telephoneline or as complex as a fiber-optic cable with
`microwave links and satellite transmissions.
`
`Private companies who make moneybyselling access to their lines build backbones, which are very high-
`capacity lines that carry enormous amountsofInternettraffic. Government agencies, such as NASA,and
`large private corporations pay for some of these backbones. The National Science Foundationalso pays for
`some backbones.
`
`19
`
`19
`
`

`

`- a
`
`.
`wy
`~
`e+TS ve
`uy
`2
`ie MN :
`- Sr
`- his
`{
`. 6
`PART | UNDERSTANDING THE INTERNET’S UNDERLYING ARCHITECTURE
`ae.
`How the Internet Can
`
`
`~
`es Together
`
`ae|
`
`Organization of networks Becausethe Internetis a loose organization of networks, no single group
`runsit or paysfor it all. Instead, many private organizations, universities, and government agencies pay
`for and run parts of It. They all work together in a democratic, loosely organizedalliance. Private organi-
`zations range from small, homegrown networks to. commercial online services, such as‘Ameriea Online
`and CompuServe,andprivate Internet service providers(ISPs) that sell access to the Internet.
`
`
`
`
`
`
`
`
`
`-
`; Ef
`
`a
`i
`
`»
`
`Funding the Internet Through agencies such as the National Science
`Foundation, the federal government pays for some high-speed backbones that
`carry Internettraffic across the country and the world. The high-speed vBNS (very
`___ high-speed Backbone NetworkServices), for‘example, provides
`yer
`a high-speedinfrastructure for the research and education
`oP
`community by linking together supercomputer centers.
`Often, a large corporation or organization such as
`eA NASA provides backbonestolink sites across the
`.
`also fundedthe Internet2, a super-fast net-
`\ work that can transfer data at an
`<.
`“astounding 2.4 gigabits per second,
`\ yy
`~~"
`for use by universities. A numberof uni-
`\Y versities already are connected toit. When
`~ finished in 2003,it will link 140 universities.
`Eventually, the entire Internet might run at its speed.
`Se
`ail
`
`country or the world. The government has
`
`yy
`
`a
`
`»
`(: aa x
`
`
`
`
`Regional networks
`Regional networks provide
`and maintain Internet access
`within a geographic area.
`Regional nets can consist of
`smaller networks and organi-
`_zations within the area that
`have banded togetherto
`providebetter service.
`
`
`
`Registrars™ Private companiescalled
`Internet registrars are responsible for
`registering Internet domains, such as
`www. zdnet.com, to people and. businesses.
`Until recently,,a quasi-public company
`called the InterNIC had‘sole responsibility
`for doing this, but other registrars can
`now register domainsas well. a
`—
`
`4
`
`Supercomputer
`Center
`
`
`
`20
`
`20
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`

`

`provi
`
`|
`
`
`
`ies are
`
`aS
`
`Internet:
`
`architectural recommendations that eo
`
`
` jice
`ISPssell peopleMont
`connectionsto the |
`ternet
`They run theirown seg- ~ Si
`
`mentsof theInternet anda
`
`also might supply long- |
`Internet
`distance connections
`
`Service
`|v
`called Papeete
`
`
`Provider
`Telephone compa
`
`~ another sourceof long-
`
`istance connections for
`
`
`nternet.
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`e@
`|
`Society
`
` Internet Society The Internet
`yen
`“x
`Society is a private nonprofit organiza- SS Sy Ae ais
`tion
`that makes rechholosiea and
`aM
`
`paren to the Internet, such as how
`
`TCP/IP.and otherInternet protocols
`|
`should work. This body guides hae
`InterNIC The InterNIC is responsible for
`
`
`
`
`direction of the Internet and its growth.
`maintaining the domains registered through
`ma
`registries. It tracks the connections
`\
`betweenInternet addresses, such as
`BS
`
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`i
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`2
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`125\34.24.21, and domain names,
`such as ww.zdnet.com.
`
`
`
`21
`
`21
`
`

`

`
`
`CHAPTER How Computer
`2?
`Networks Send
`Data Across
`the Interne
`
`A
`
`22
`
`22
`
`

`

`you might take for granted that when you sendapiece of information across the Internet, it will always
`reach its intended destination. However, the process of sending that information is remarkably complex.
`Whenyou send information across the Internet, the Transmission Control Protocol (TCP)—the language
`computers use when communicating over the Internet—first breaks the information up into packets, smaller
`blocks of information that also contain a variety of data that helps the packets travel across the Internet.
`Your computer sends those packets to your local network, Internet service provider (ISP), or online service.
`From there, the packets travel through manylevels of networks, computers, and communicationslines
`before they reach their final destinations, which might be across town or around the world. A variety of
`hardware processes those packets and routes them to their proper destinations. This hardware is designed to
`transmit data between networks and makes up muchofthe glue that holds the Internet together. Five of the
`most important pieces of hardware are hubs, bridges, gateways, repeaters, and routers.
`Hubs are important because they link groups of computers to one another and let computers communi-
`cate with each other. Bridges link local area networks (LANs) with one another. They enable data destined
`for another LAN to be sent there, while keeping local data inside its own network. Gateways are similar to
`bridges, but they also translate data from one type of network to another.
`Whendatatravels across the Internet,it often crosses great distances, which can be a problem because
`the signal sending the data can weaken overthe distance. To solve the problem, repeaters amplify the data at
`intervals so the signal doesn't weaken.
`Routers play a key role in managing Internet traffic. Their job is to ensure the packets alwaysarrive at
`the proper destination. If data is being transferred among computers that are on the same LAN, routers
`often aren’t necessary because the network itself can handle its internaltraffic. Routers come into play when
`the data is sent between two different networks. Routers examine packets to determine their destinations.
`They take into account the volumeof activity on the Internet, and they send the packet to another router
`that is closer to the packet's final destination. For more information on routers see Chapter 6, “How Routers
`Work.”
`
`All this hardware connects the many networks that make up the Internet. At the most local level of net-
`works are corporate local area networks. Midlevel networks hook these LANstogether using high-speed
`telephone lines, ethernet, and microwavelinks, A regional network is a midlevel network in a geographic
`area. A wide area network (WAN) is another type of midlevel network. A WAN consists of an organization
`with many networkedsites linked together.
`
`Whena packettravels from a computer on a LAN in a midlevel network to a computer somewhereelse
`on the midlevel network, a router (or a series of routers) sends the packet to its proper destination. However,
`if the destination lies outside the midlevel network, the packet is sent to a network access point (NAP),
`whereit is sent across the country or the world on a backbone. High-speed backbones such as the vVBNS
`(very high-speed Backbone Network Services) can transmit enormous amounts of data—155 megabits (mil-
`lions of bits) per second (Mbps). Even faster backbones are being built that will transmit data at an astonish-
`ing 9.6 billion bits per second.
`
`23
`
`23
`
`

`

`
`
`ow Netw
`
`to the Interne
`
`
`iy
`
`—
`—
`
`Dial Up
`
`t.
`
`~~
`
`.
`
`{
`
`You get onto the Internet through a LAN at
`your homeorplace of businessin one of two
`ways. You can dial into a large computer con-
`nected to the Internet via an online service ora
`dial-in Internet service provider (ISP), or you can
`use anothertype of Internet service, such as a
`cable modem. Ethernet networks and token-ring
`networks are two types of networksthat
`
` can be connected to the Internet. Token-
`
`ring networks pass data in tokens from
`computer to computer in a ring or star
`configuration. In Ethernet networks,
`the data goes from a server
`to a computer on the
`
`Satellite
`
`
`Link
`
`Ethernet Network [o>
`——
`
`OF
`
`Server
`
`Token-Ring Network
`
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`ez
`Ty
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`By, /
`
`id
`network.
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`>ile
`F
`
`X
`a
`
`iter you're connected
`
`
`and you want to-send or
`
`your @2a
`receive information;
`requests anddata are
`a
`
`handled by routers on the #
`Internet. Routers perform *
`
`mostof the workof direct-
`ing traffic on the Internet.
`They examine the packets of data that travel acrossthe Internet to
`see where the data is headed. Based on the data’s destination, the
`packet is routed in the mostefficient way—generally to another
`router, which in turn sends the packet to the next router, and so on.
`Routers also connect networksto each other.
`
`E The data can be transferred between networks in a numberof ways. Dedicated telephonelines can transmit
`data at 56Kbps (kilobits per second). An increasing number of T1 leased telephone lines carry data between
`networks. A T1 link can carry data at 1.544Mbps. Higher-speed T3 links, which can carry data at 44.746Mbps,
`are being usedas well. If you dial into an ISP from home, you might connect at a lower speed than at your
`office, where you might have higher-speed connections such as a T1 or 56Kbpsline. However, a variety of
`high-speed options are available for connecting to the Internet at home, such as cable modemsor high-
`speed Digital Subscriber Line (DSL) connections.
`
`24
`
`24
`
`

`

`
` CHAPTER 2HOWCOMPUTER
`
`4|A variety of other methods transfer J
`
`
`data between networks. Satellites can
`be used to send and receive informa-
`tion, as can fiber-optic cables, spe-
`cial Integrated Services Digital
`Network (ISDN) telephone
`lines, and high-speed
`
`Satellite
`
`DSL connections.
`Link
` GB The networksin a particular geo-
`
`graphic area are connected into a
`large regional network. Routers
`pass information within that <>
`area from network to network
`
`ys,
`
`
`
`
`
`Gg Regional networks are connected to one anothervia
`high-speed backbones—connections that can send
`
`data at extremely high speeds. Whendatais sent from
`one regional network to another,it is first sent to a net-
`
`
`work access point (NAP). The NAP then routes the data
`
`to high-speed backbones, such as vBNS, which can
`transmit data at 155Mbps. The data is then sent
`along the backbone to another regional net-
`
`work.Finally, itis passed toaspecific
`
`network and computer within that
`regional network.
`
`
`
`25
`
`25
`
`

`

`
`
`cee How TCP/IP
`3
`Works
`
`
`
`26
`
`26
`
`

`

`A seemingly simple set of ideas makesit possible for computers and networksall over the world to share
`information and me