Cisco SYSTEMS
`
`
`
`Computer Networking
`Essentials
`
`An essential guide to understanding networking
`theory, implementation, and interoperability
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`; Computer Networking Essentials
`DebraLittlejohn Shinder
`
`CiscoPress
`o
`Cisco Press —
`201 West 103rd Street
`Indianapolis, IN 46290 USA
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`Computer Networking Essentials
`Debra Littlejohn Shinder
`Copyright © 2002 Cisco Systems, Inc.
`Published by:
`Cisco Press
`201 West 103rd Street
`Indianapolis, IN 46290 USA
`All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic
`or mechanical, including photocopying, recording, or by any information storage and retrieval system, without
`written permission from the publisher, except for the inclusion of brief quotations in a review.
`Printed in the United States of America 34567890
`Third Printing
`January 2002
`Library of Congress Cataloging-in-Publication Number: 2001090429
`ISBN: 1-58713-038-6
`
`Trademark Acknowledgments
`All terms mentionedin this book that are known to be trademarksor service marks have been appropriately capital-
`ized. Cisco Press or Cisco Systems, Inc., cannotattest to the accuracy of this information. Use of a term in this book
`should not be regardedas affecting the validity of any trademark or service mark.
`
`Warning and Disclaimer
`This bookis designed to provide information about basic networking and operating system technologies. Every
`effort has been madeto make this book as complete and as accurate as possible, but no warranty orfitness is
`implied.
`The information is provided on an “as is” basis. The author, Cisco Press, and Cisco Systems,Inc., shall have neither
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`contained in this book or from the use ofthe discs or programs that may accompany it.
`The opinions expressed in this book belongto the authorand are not necessarily those of Cisco Systems,Inc.
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`
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`All otherbrands, names,or trademarks mentioned in this document or Web site are the property oftheir respective owners. The use of the word partner does not imply a partnership
`relationship between Cisco and any other company. (0010R)
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`
`Introduction
`Computer Networking Essentials helps you understand the fundamentals of computer networking concepts and
`implementation and introduces youto the client and server operating systems that run on networked PCs.
`Concepts covered in this book include the history of networking, networking terminology, networking theory and
`established standards, and implementation of local-area and wide-area networks. Special emphasis is placed on
`understanding network protocols and how they operateatall layers of the networking model. Emphasisalso is
`placed on the interoperability of networks that run on multiple protocols, platforms, and operating systems.
`Specialty areas such as security, remote access, virtual private networking, thin client networking, monitoring, man-
`agement, and troubleshooting are covered thoroughly. Emerging technologies that are expected to impact the future
`of networking are also introduced.
`
`WhoShould Read This Book
`This book’s primary audienceis professionals who are beginning training in the networking industry and those who
`need a review of basic concepts.
`The secondary audience includes corporate training faculties and staff and members of the business world who
`work with information technology personnel and require a broad overview ofthe concepts involved in networking
`from the small business to the enterprise-level corporation.
`A third target audienceis the general user who wants to know more about how computers communicate over net-
`works. The book’s approachis designedto be user-friendly and accessible to the non-technical reader whois
`overwhelmed by the jargon found in vendor documentation andtechnical manuals.
`
`This Book’s Organization
`This bookis organized into four parts and includes 19 chapters, an appendix,and a glossary. The following sections
`describe the contents of each part of the book.
`
`PartI: Introduction to Networking Concepts
`Chapter 1, “Introduction to PC Networking,” introduces you to the basic concepts of PC networking by providing a
`brief history of electronic communications and networking and a summary of where PC networkingis today.
`Chapter2, “Categorizing Networks,” discusses the categorization of networks according to physical scope, admin-
`istrative model, network operating system, protocols in use, topology, and architecture.
`Chapter 3, “Networking Concepts, Models, and Standards,” provides an overview of binary communications and
`introduces two popular networking models: the Department of Defense (DoD) model on which the TCP/IP proto-
`cols are based and the Open Systems Interconnection (OSI) model, which was developed by the International Orga-
`nization for Standardization (ISO). Specifications set forth by the Institute of Electrical and Electronics Engineers
`(IEEE) and vendor-specific models are also covered.
`Chapter4, “Networking Communications Methods,”discusses signaling methods and provides an understanding of
`analog, digital, broadband, baseband, asynchronous, synchronous, simplex, duplex, and multiplexed signaling.
`Media access methods are described, including CSMA/CD, CSMA/CA,token passing, and demandpriority.
`Chapter 5, “LANLinks,” discusses popular LAN types, including Ethernet, Token Ring, FDDI, AppleTalk, and
`ARCnet.
`Chapter 6, “WAN Links,” provides an overview of WANconnections such as PSTN, ISDN,t-carriers, Frame Relay,
`X.25, and CATV network, as well as high-speed connectivity solutions such as ATM, SONET,and SMDS. This
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`chapter also covers LAN-to-WANconnection solutions, including Internet Connection Sharing (ICS), Network
`Address Translation (NAT), proxy servers, and routed connections.
`
`Part II: Networking Hardware and Software
`Chapter 7, “Physical Components of the Network,” introduces students to the many types of networking media,
`including coax, twisted-pair cable, and fiber-optic cable, as well as to wireless technologies such aslaser, infrared,
`radio, and satellite/microwave communications. Connectivity devices such as repeaters, hubs, bridges, routers, and
`switchesare also discussed.
`
`Chapter8, “Networking Protocols and Services,” describes common LAN protocols—TCP/IP, NetBEUI, IPX/
`SPX—and discusses the OSI protocol suite. PPP and SLIP, which are WANlink protocols, and PPTP and L2TP,
`which are common tunneling protocols, are also presented.
`Chapter 9, “The Widest Area Network: The GlobalInternet,” discusses the evolution of the Internet, the protocols
`used for Internet communications—HTTP, FTP, NNTP, SMTP, and POP—and the TCP/IP protocolsuite.
`Chapter 10, “Network Operating Systems,” discusses general network administration practices and then looksat the
`specifics of commonserver operating systems, including Windows NT, Windows 2000, NetWare, UNIX, and Linux.
`Chapter 11, “Directory Services,” describes the Directory Services Protocol (DAP) and the Lightweight Directory
`Access Protocol (LDAP), as well as the X.500 standards developed by the ISO to promote directory services com-
`patibility and interoperability. Novell’s NDS, Microsoft’s Active Directory, and Banyan VINES’ StreetTalk direc-
`tory services are covered in some depth.
`Chapter 12, “Desktop Operating Systems,” looksat the client side of the client/server network and discusses the
`advantages and disadvantages of common desktopclients, such as DOS, Windows, Linux, Macintosh, and OS/2,
`and how each can be integrated into popular NOS environments.
`Chapter 13, “Hybrid Networks,” provides information about interoperability solutions and protocol gateways that
`allow PCs running different operating systems, protocols, and platforms to communicate with one another. This
`chapteralso looks at PC-to-mainframe communications using Systems Network Architecture (SNA) solutions.
`
`Part III: Network Specialty Areas
`Chapter 14, “Protecting the Network,” addresses security issues and provides an overview of basic cryptography
`concepts, public and private key encryption, certificate services, firewalls and proxies, and internal security mea-
`sures such as “smart cards” and advanced authentication technologies.It also provides guidance for developing
`security policies for your network. The secondhalf of the chapter discusses disaster recovery plans, including
`implementation of disk fault tolerance (or RAID), regular scheduled backups, and server
`clustering.
`Chapter 15, “Remote Access,” discusses methods of connecting to a server from a remotelocation using remote
`connectivity devices such as modems, ISDN terminal adapters, and customer premises equipment (CPE) for dedi-
`cated lines. Dial-in server configuration and special security considerations are also covered.
`Chapter 16, “Virtual Private Networking,” provides an overview of VPN concepts and discussesthe tunneling pro-
`tocols used to provide VPN security.
`Chapter 17, “Thin Client Networking,” discusses Network Computers, Net PCs, and Windows-based terminals.
`Windowsterminal services, Citrix Metaframe, web-based computing, the X Window system and Javavirtual
`machines—andthe role each plays in thin client networking—arealso discussed.
`Chapter 18, “Monitoring, Management, and Troubleshooting Tools,” presents an introduction to the TCP/IPutilities
`and other tools built into the various operating systems. This chapter also examines commercial products such as
`Sniffer Pro, LANanalzyer, Microsoft’s Systems Management Server, Novell’s ManageWise, and IBM’sTivoli.
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`Part IV: The Future of Networking
`Chapter 19, “Tomorrow’s Technologies,” takes a look into the future of PC networking. It discusses ways of over-
`comingthe currentlimits of IP, including the new version of IP—IPv6. The goal of universal connectivity is
`addressed, and more exotic possibilities such asartificial intelligence, quantum computing, and cyberneticlife
`formsare presented as possible components of tomorrow’s networks.
`
`This Book’s Features
`This book contains several elements that help you learn about operating systems and networking:
`Figures,listings, and tables—This book containsfigures,listings, and tables that help to
`explain concepts, commands,and procedural sequences. Diagramsillustrate network layouts
`and processes, and screenshots assist students in visualization configuration procedures. In addi-
`tion,listings and tables provide summaries and comparisonsof features and characteristics.
`Author’s notes,tips, sidebars, and cautions—These elements are included to provide you
`with extra information on a subject. You will probably find these asides to be very beneficial in
`real-world implementations.
`Chapter summaries—Atthe end of each chapteris a summary of the concepts covered in the
`chapter, which provides a synopsis of the chapter and canserve as a study aid.
`Further Reading—Eachchapter includesa list of resources for additional information about
`the topics covered in the chapter, including website URLs and booksandarticles that cover the
`topic in more detail.
`Review questions—After the Further Reading section in each chapter are 10 review questions
`that serve as an end-of-chapter assessment. The questions are designed to reinforce the con-
`cepts introducedin the chapter andto help students evaluate their understanding before moving
`onto the next chapter.
`The conventionsused to present command syntax in this book are the same conventions used in the Cisco IOS
`Command Reference, as follows:
`Boldface indicates commands and keywordsthatare enteredliterally as shown. In examples
`(not syntax), boldface indicates user input (for example, a show command).
`Italics indicates arguments for which you supply values.
`Square brackets [ ] indicate optional elements.
`Vertical bars(I) separate alternative, mutually exclusive elements.
`Braces and vertical bars within square brackets—for example,[x {y | z}|—indicate a required
`choice within an optional element. You do notneedto enter what is in the brackets, but if you
`do, you have some required choices in the braces.
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` CHAPTER
`
`Introduction to PC Networking
`
`
`
`Welcometo the world of personal computer (PC) networking.In this world,it is no longer
`enough to simply have and use PCs; todayit is imperative that you also “get connected.”
`The real power and usability of PCs becomes apparent only when they are linked so that
`they can communicate with one another. From the simple two-computer homeor small
`office local-area network (LAN)to the ever-growing global Internet, networkingis the
`future of computing, and that future is here today.
`
`In manyareas of the United States, the demandfor trained networking professionals far
`exceeds the supply. According to projections of the U.S. Department of Labor, computer
`networking as an occupation has a bright future. Businesses and individuals are buying
`PCs, and those computers are linking within LANs and wide-area networks (WANs)at an
`astonishing pace. We literally are “networking the world.”
`
`Because network communications is quickly becoming a part of ourlives, even those not
`directly involved in the information technology (IT) industry should know something about
`the basics of networking. Just as it would be difficult to function in today’s world if you
`knew nothing about a telephoneandits features, in the not-too-distant future, knowing how
`to “get on the network” will be a requirement for many individuals, both at work and at
`home.
`
`
`
`
`
`ABrie History of PC Networking
`
`The desire to communicate with others is a driving force among human beings, and the
`sophisticated means we have developed to communicate sets us apart from other species.
`From the momentit became possible to link two computers and get them to talk to one
`another, the concept of the Internet was inevitable.
`
`In the early days of computing, computers were enormous machinesthatfilled entire
`rooms—sometimes entire city blocks—and cost hundreds of thousandsof dollars.
`Although these expensive behemoths had less processing power and memory than today’s
`tiny handheld computers, they were state-of-the-art technology in the 1950s and 1960s. In
`a world in which human beings who were slow and prone to error had done calculations
`manually, the capabilities of the computer were amazing.
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`Chapter 1: Introduction to PC Networking
`6
`
`
`At the midpoint of the twentieth century, computers were still rare, exotic, mysterious
`machines ownedonly by large companies, governmental bodies, and educational
`institutions. For the most part, computers were standalone systems, isolated from one
`another.
`In the 1940s, Thomas Watson, the chairman of IBM,said that a market existed in the world
`for approximately five computers. Even as recently as 1977, Ken Olson,presidentofDigital
`Equipment Corporation,said, “There is no reason anyone would want a computer in their
`home” (ISBC[International Small Business Consortium], www.isbc.com/isbe/business/
`wisdom.cfm). Of course, both have been proven not just wrong, but very wrong. However,
`no one would havepredicted, even a decade ago, that PCs wouldproliferate as they have or
`that computer networking would become a mainstream topic.
`
`The First Communications Networks
`By the mid-1900s,electronic communications had been aroundfor over a century and was
`being implemented in both Europe andthe United States. These early networks took many
`formsand sent only codedsignals. They later became capable of sending voice across the
`wire.
`This section provides a roughtime line of how thefirst networks were developed.
`
`Telegraph Cables
`In the early 1800s, the French developedthefirst optical telegraph network, which sent
`information at the blazing speed of 20 characters per second, and Samuel Morse
`demonstratedtheelectrical telegraph, which spurred the developmentof networked
`communications in the United States.
`
`The Telephone Network: Circuit-Switching Technology
`In the late 1800s, a vast telephone network began to be built. Technology leaders of the day,
`however, were no morefarsighted than those of the early computer age. In 1876 an internal
`memoat Western Unionstated that “This ’telephone’ has too many shortcomings to be
`seriously considered as a meansof communication. The device is inherently of no value to
`us” (www.isbc.com/isbc/business/wisdom.cfm).
`Despite thatattitude, there were more than 50,000 telephonelinesin the U.S. by 1880, and
`by 1960, telephone lines covered urbanareas, and the telephone network became a global
`communications network.
`A telephone system uses circuit-switching technology,in which a circuit, or virtual
`pathway,is established when onetelephone connects to another on a network. This works
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`
`Figure 1-1
`
`well for voice transmission because the sounds being transferred over the wire flow at a
`relatively constantrate.
`
`In a circuit-switched network, a connection is established, as shown in Figure 1-1. All
`signals are passed overthis circuit for the duration of the session. If you disconnect and
`reconnect, a different circuit can be used, as represented by the dottedline.
`
`In a circuit-switched network, a connection is established, as represented by the solid line.
`
`
`
`The technology works less well for transfer of computer data, which has a tendency to be
`sent in bursts; that is, periods of high activity are interspersed with intervals of low activity
`or inactivity.
`
`Packet-Switching Technology
`During the 1960s, the U.S. government becameinterested in developing a computer
`network that would enable systemsat military installations and major educational
`institutions to communicate with one another. Because this was during the middle of the
`Cold War, they wanted the network to have robustness, reliability, and redundancyso that
`the network would survive a nuclear war.
`
`Researchers working at the Massachusetts Institute of Technology (MIT), the RAND
`Institute, and the National Physical Laboratory (NPL) in England invented a new
`technology called packet switching, which worked better for bursty transmissions than did
`the traditional circuit-switching technologies. Their work created a foundation for the
`communications technology used on the Internet today.
`
`In a packet-switched network, as shown in Figure 1-2, a connection is not established for
`the entire transmission. Instead, each individual packet of data can take a different path.
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`8 Chapter 1: Introduction to PC Networking
`
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`Communications from different sources can share the sameline, rather than the line being
`dedicated to one end-to-end communicationfor the duration of a session, as is the case with
`circuit switching.
`
`Figure 1-2 Networked computers share data, software, and hardware resources.
`
`
`
`Circuit Switching Versus Packet Switching
`The terms circuit switching and packet switching soundalike but have different meanings.
`Thepublic telephone system, sometimesreferredto as POTS(plain old telephone service),
`is a switched-circuit communications network. Whenyouplace a telephonecall in this type
`of network, only one physical path from your telephone to the one you’re dialing is used
`for the duration of that call. This pathway, or circuit, is maintained for your exclusive use,
`until you end the connection by hanging up your telephone.
`Note, however, that if you call the same friend at the same number tomorrow,and do so at
`the same location from which you placed today’s call, the path is not necessarily the same.
`That’s whythe circuit is referred to as switched.It also explains why you can get a clear
`connection one day and noiseandstatic on another.
`With a packet-switching network, no dedicated pathway or circuit is established. Packet
`switching is sometimesreferred to as a connectionless technology because ofthe lack of a
`dedicated pathway.If you transfer data, such as a word processing file, from your computer
`to another using a packet-switched network, each individual packet (that is, each small
`chunkof data) can take a different route. Although it all arrives at the same destination,it
`doesn’tall travel the samepath to get there. Internettraffic generally uses packet-switching
`technology.
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`A Brief History of PC Networking
`
`9
`
`The difference between circuit and packet switching can be comparedto the different ways
`in which a large group of people traveling from Dallas to San Francisco can reach their
`destination. For example, circuit switching is similar to loading the entire group on a bus,
`a train, or an airplane. Theroute is plotted out, and the whole group travels over that same
`route.
`
`Packet switching is like having each person travel in an automobile. The group is broken
`downinto individual componentsas the data communication is broken into packets. Some
`travelers can take interstate highways, and others can use back roads. Some can drive
`straight through, and others can take a more roundaboutpath. Eventually, they all end up at
`the same destination. The group is put back together, just as packets are reassembledat the
`endpoint of the communication.
`
`The ARPAnet
`The first packet-switched computer network was conceived in the late 1960s, under the
`auspices of the U.S. Department of Defense (DoD). It was christened the ARPAnet(for
`Advanced Research Projects Agency network). The ARPAnet’s first node, or connection
`point, was installed at the University of California at Los Angeles in 1969. In just three
`years, the network spread across the United States, and two years after that, it spread to
`Europe.
`
`As the network grew,it split into two parts. The military called its part of the internetwork
`Milnet, and ARPAnet continuedto be used to describe the part of the network that
`connected research and university sites. In the 1980s the Defense Data Network (a separate
`military network) and NSFNet(a network of scientific and academic sites funded by the
`National Science Foundation) replaced ARPAnet. Eventually this WAN grew into what we
`today call the Internet.
`
`Yesterday’s Networks
`Computer networking didn’t begin on such a large scale as the ARPAnetproject; that is, the
`LANcamebefore the WAN. As computers becameless expensive and more powerful,
`businesses ofall sizes more commonly used them. Althoughthefirst machines were useful
`for only very limited types of data processing, as software developmentflourished, new
`programs enabled users to do much morethan just collect and sort data.
`
`With early mainframe systems, for instance, multiple users could access the same stored
`data from terminals, which were stations with input and output devices (for example,
`keyboards and monitors). These stations had no computing powerof their own; they were
`points from which the mainframe computer could be accessed.
`
`Using mainframes worked well in many respects, but they had several disadvantages when
`compared to smaller computers (then called microcomputers). Expense was one
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`disadvantage; large mainframe systemscost far more than the so-called “personal”
`computers designedto sit on a desktop and function independently.
`Another disadvantage of mainframes was the single point offailure concept. With
`mainframe computing, ifthe computer was down,it was downfor everyone. Nobody could
`access data, and nobody who depended on the computer could get any work done. The use
`of individual PCs, on the other hand, circumventedthis problem.
`PCs were full-fledged computers that ran programs and performedtasksentirely on their
`own. Theyalso provided some measure offault tolerance, whichis the capability of a
`system to continueto function and ensure data integrity when failures occur. If one
`employee’s computer crashed,it didn’t affect the capability of the rest of the employees,
`who had their own PCs, to continue working. In fact, if an employee had saved data to a
`floppy disk, he or she could move to a functioning machine and continue working.
`These factors contributed to the increased popularity of PCs as a computing solution for
`small and large businesses (and everything in between). However, once everyone had aPC
`on the desktop, companies were faced with a dilemma: How could workers share
`information as they had with the old mainframe computing model? The solution was
`networking.
`
`©
`
`Disadvantages of Standalone Systems
`In the early days of desktop PCs, networking hardware and software were not readily
`available, and many businesses used the machinesas standalone systems.If all users
`needed to print documents on occasion, there were three possible ways to provide that
`ability:
`© A printer could be attached to each machine. This was a costly solution becauseit
`necessitated buying multiple printers, even though it was unlikely that they all would
`be in use at the same time.
`Thefile to be printed could be saved to floppy disk and transferred to a machine that
`had an attachedprinter. This was a less-expensive option, but it was an inconvenience
`both to the person whohad to go begging fora printer and to the person with the
`printer, whose work was interrupted while someone else used his or her machine to
`print.
`© A printer could be moved from one workstation to another, depending on who needed
`to print. This was a somewhat cumbersomesolution; nonetheless, it was widely
`implemented,using rolling printer carts that were wheeled aroundthe office. Each
`move required that cables be disconnected and reconnected, and sometimes, a move
`involved software reconfiguration as well.
`High cost, inconvenience, and extra work are the primary disadvantages of standalone, or
`non-networked, solutions.
`
`Facebook's Exhibit No. 1008/1108
`Page 13
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`Facebook's Exhibit No. 1008/1108
`Page 13
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`

`

`11
`A Brief History of PC Networking
`
`
`What Is a Network, Anyway?
`The American Heritage Dictionary defines a network as “a system of lines or channels that
`cross or interconnect.” Earlier we mentioned the telegraph and telephone networks, and of
`course, we’ve all heard references to the television networks. Using the dictionary
`definition, we can call even the state highway system, or the railways that crisscross the
`country, a network.
`That being said, what is a computer network? Simply,it is two or more devices linked for
`the purpose of sharing information, resources, or both. The link can be through cable
`(coaxial, twisted-pair, or fiber optics, as you’ll learn later in this chapter), or it can be a
`wireless connectionthat uses radio signals, laser or infrared technology, orsatellite
`transmission. The information and resources shared can bedatafiles, application programs,
`printers, modems,or other hardware devices. See Figure 1-3 for anillustration.
`
`Figure 1-3=This time line shows significant events in PC networkinghistory.
`
`ny
`
`Workstation
`
`Workstation
`
`Workstation
`
`Server
`
`
`
`Printers
`
`Files Applications Modems
`
`Scanners
`
`Why Network Computers?
`If the advantage of PCs were each user having an independent computer, why would we
`want to turn around and link them again? We link them because networked PCsgive us, in
`many ways, the best of both worlds. Each user has independentprocessing power,butstill
`can enjoyall the benefits of sharing. On the other hand, a companyseesa significant cost
`savings when expensive, occasionally used peripherals are shared over the network. For
`example, an expensive colorlaser printer might be used only for special projects, yet many
`
`Facebook's Exhibit No. 1008/1108
`Page 14
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`Facebook's Exhibit No. 1008/1108
`Page 14
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`

`

`Chapter1: Introduction to PC Networking
`42
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