Clsco SYSTEMS
`
`
`
`Computer Networking
`Essentials
`
`An essential guide to understanding networking
`theory, implementation, and interoperability
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`Computer Networking Essentials
`
`DEbt‘yaiL‘ititflejohn Shinder
`
`Cisco: Press
`
`r
`Cisco Press L
`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 3 4 5 6 7 8 9 0
`
`Third Printing
`January 2002
`Library of Congress Cataloging—in-Publication Number: 2001090429
`ISBN: l-587l3~038—6
`
`Trademark Acknowledgments
`All terms mentioned in this book that are known to be trademarks or service marks have been appropriately capita1~
`ized. Cisco Press or Cisco Systems, Inc., cannot attest 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
`This book is designed to provide information about basic networking and operating system technologies. Every
`effort has been made to make this book as complete and as accurate as possible, but no warranty or fitness is
`implied.
`The information is provided on an “as is” basis. The author, Cisco Press, and Cisco Systems, Inc., shall have neither
`liability nor responsibility to any person or entity with respect to any loss or damages arising from the information
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`The opinions expressed in this book belong to the author and are not necessarily those of Cisco Systems, Inc.
`
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`
`
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`All other brands, names, or trademarks mentioned in this document or Web site are the property of their respective owrters. The use of the word partner does not imply a partnership
`relationship between Cisco and any other company. (OOlOR)
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`
`Introduction
`Computer Networking Essentials helps you understand the fundamentals of computer networking concepts and
`implementation and introduces you to 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 operate at all layers of the networking model. Emphasis also 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.
`
`Who Should Read This Book
`This book’s primary audience is 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 of the concepts involved in networking
`from the small business to the enterprise—level corporation.
`
`A third target audience is the general user who wants to know more about how computers communicate over net-
`works. The book’s approach is designed to be user—friendly and accessible to the non-technical reader who is
`overwhelmed by the jargon found in vendor documentation and technical manuals.
`
`This Book’s Organization
`This book is 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.
`
`Part I: 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 networking is today.
`Chapter 2, “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 (081) 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.
`Chapter 4, “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 demand priority.
`Chapter 5, “LAN Links,” discusses popular LAN types, including Ethernet, Token Ring, FDDI, AppleTalk, and
`ARCnet.
`
`Chapter 6, “WAN Links,” provides an overview of WAN connections such as PSTN, ISDN, t—carriers, Frame Relay,
`X25, 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-WAN connection solutions, including Internet Connection Sharing (ICS), Network
`Address Translation (NAT), proxy servers, and routed connections.
`
`Part 11: 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 as laser, infrared,
`radio, and satellite/microwave communications. Connectivity devices such as repeaters, hubs, bridges, routers, and
`switches are also discussed.
`
`Chapter 8, “Networking Protocols and Services,” describes common LAN protocols—TCP/IP, NetBEUI, IPX/
`SPX—and discusses the OSI protocol suite. PPP and SLIP, which are WAN link protocols, and PPTP and LZTP,
`which are common tunneling protocols, are also presented.
`
`Chapter 9, “The Widest Area Network: The Global Internet,” discusses the evolution of the Internet, the protocols
`used for Internet communicationsmHTTP, FTP, NNTP, SMTP, and POP—and the TCP/IP protocol suite.
`
`Chapter 10, “Network Operating Systems,” discusses general network administration practices and then looks at the
`specifics of common server 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 X500 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,” looks at the client side of the client/server network and discusses the
`advantages and disadvantages of common desktop clients, such as DOS, Windows, Linux, Macintosh, and 05/2,
`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
`chapter also looks at PC—to—mainfrarne 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 second half 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 remote location 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 discusses the tunneling pro—
`tocols used to provide VPN security.
`
`Chapter 17, “Thin Client Networking,” discusses Network Computers, Net PCs, and Windows-based terminals.
`Windows terminal services, Citrix Metaframe, web-based computing, the X Window system and Java virtual
`machines—and the role each plays in thin client networking—are also discussed.
`
`Chapter 18, “Monitoring, Management, and Troubleshooting Tools,” presents an introduction to the TCP/IP utilities
`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’s Tivoli.
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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-
`coming the current limits of IP, including the new version of IP——-1Pv6. The goal of universal connectivity is
`addressed, and more exotic possibilities such as artificial intelligence, quantum computing, and cybernetic life
`forms are 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 contains figures, listings, and tables that help to
`explain concepts, commands, and procedural sequences. Diagrams illustrate network layouts
`and processes, and screenshots assist students in visualization configuration procedures. In addi-
`tion, listings and tables provide summaries and comparisons of features and characteristics.
`Author’s notes, tips, sidebars, and cautionsflThese 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—At the end of each chapter is a summary of the concepts covered in the
`chapter, which provides a synopsis of the chapter and can serve as a study aid.
`Further Reading-Each chapter includes a list of resources for additional information about
`the topics covered in the chapter, including website URLs and books and articles 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 introduced in the chapter and to help students evaluate their understanding before moving
`on to the next chapter.
`The conventions used to present command syntax in this book are the same conventions used in the Cisco IOS
`Command Reference, as follows:
`Boldface indicates commands and keywords that are entered literally 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 bracketswfor example, [x {y | z}]-——indicate a required
`choice within an optional element. You do not need to 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
`
`Welcome to the world of personal computer (PC) networking. In this world, it is no longer
`enough to simply have and use PCs; today it 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 home or small
`office local—area network (LAN) to the ever-growing global Internet, networking is the
`future of computing, and that future is here today.
`
`In many areas of the United States, the demand for trained networking professionals far
`exceeds the supply. According to projections of the US. 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 our lives, 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 telephone and its 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.
`
`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 moment it 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 machines that filled entire
`rooms—sometimes entire city blocks—and cost hundreds of thousands of 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 19603. 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.
`
`
`
`E‘EA-B‘rief
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`At the midpoint of the twentieth century, computers were still rare, exotic, mysterious
`machines owned only 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, president of Digital
`Equipment Corporation, said, “There is no reason anyone would want a computer in their
`home” (ISBC [International Small Business Consortium], www.isbc.com/isbc/business/
`wisdomcfm). Of course, both have been proven not just wrong, but very wrong. However,
`no one would have predicted, even a decade ago, that PCs would proliferate as they have or
`that computer networking would become a mainstream topic.
`
`The First Communications Networks
`By the mid—1900s, electronic communications had been around for over a century and was
`being implemented in both Europe and the United States. These early networks took many
`forms and sent only coded signals. They later became capable of sending voice across the
`wire.
`
`This section provides a rough time line of how the first networks were developed.
`
`Telegraph Cables
`In the early 1800s, the French developed the first optical telegraph network, which sent
`information at the blazing speed of 20 characters per second, and Samuel Morse
`demonstrated the electrical telegraph, which spurred the development of 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 more farsighted than those of the early computer age. In 1876 an internal
`memo at Western Union stated that “This ’telephone’ has too many shortcomings to be
`seriously considered as a means of communication. The device is inherently of no value to
`us” (www.isbc.com/isbc/business/wisdom.cfm).
`Despite that attitude, there were more than 50,000 telephone lines in the US. by 1880, and
`by 1960, telephone lines covered urban areas, 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 one telephone connects to another on a network. This works
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`A Brief History of PC Networking
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`well for voice transmission because the sounds being transferred over the wire flow at a
`relatively constant rate.
`
`In a circuit~switched network, a connection is established, as shown in Figure 1-1, All
`signals are passed over this circuit for the duration of the session. If you disconnect and
`reconnect, a different circuit can be used, as represented by the dotted line.
`
`Figure 1-1
`
`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 US. government became interested in developing a computer
`network that would enable systems at 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 redundancy so 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 (NFL) 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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`Communications from different sources can share the same line, rather than the line being
`dedicated to one end—to—end communication for 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 sound alike but have different meanings.
`
`The public telephone system, sometimes referred to as POTS (plain old telephone service),
`is a switched~circuit communications network. When you place a telephone call 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 why the circuit is referred to as switched. It also explains why you can get a Clear
`connection one day and noise and static on another.
`
`With a packet-switching network, no dedicated pathway or circuit is established. Packet
`switching is sometimes referred to as a connectionless technology because of the 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
`chunk of data) can take a different route. Although it all arrives at the same destination, it
`doesn’t all travel the same path to get there. Internet traffic 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 compared to 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. The route 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
`down into individual components as 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 roundabout path. Eventually, they all end up at
`the same destination. The group is put back together, just as packets are reassembled at the
`endpoint of the communication.
`
`The ARPAnet
`
`The first packet-switched computer network was conceived in the late 19605, under the
`auspices of the US. 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 continued to 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 ARPAnet project; that is, the
`LAN came before the WAN. As computers became less expensive and more powerful,
`businesses of all sizes more commonly used them. Although the first machines were useful
`for only very limited types of data processing, as software development flourished, new
`programs enabled users to do much more than 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 power of 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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`10 Chapter 1: Introduction to PC Networking
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`
`disadvantage; large mainframe systems cost far more than the so~called “personal”
`computers designed to sit on a desktop and function independently.
`Another disadvantage of mainframes was the single point offailure concept. With
`mainframe computing, if the computer was down, it was down for 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, circumvented this problem.
`PCs were full~fledged computers that ran programs and performed tasks entirely on their
`own. They also provided some measure offault tolerance, which is the capability of a
`system to continue to 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 a PC
`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 machines as 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 because it
`necessitated buying multiple printers, even though it was unlikely that they all would
`be in use at the same time.
`° The file to be printed could be saved to floppy disk and transferred to a machine that
`had an attached printer. This was a less—expensive option, but it was an inconvenience
`both to the person who had to go begging for a 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 cumbersome solution; nonetheless, it was widely
`implemented, using rolling printer carts that were wheeled around the 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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`

`

`
`
`A Brief History of PC Networking 11
`
`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 connection that uses radio signals, laser or infrared technology, or satellite
`transmission. The information and resources shared can be data files, application programs,
`printers, modems, or other hardware devices. See Figure 1—3 for an illustration.
`
`Figure 1-3
`
`This time line shows significant events in PC networking history.
`
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`
`
`
`Workstation
`
`Workstation
`
`Workstation
`
`Server
`
`
`
`Files Applications Modems
`
`Scanners
`
`Printers
`
`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 PCs give us, in
`many ways, the best of both worlds. Each user has independent processing power, but still
`can enjoy all the benefits of sharing. On the ot