Networking the complete reference, third edition bobbi sandberg

Networking the complete reference, third edition bobbi sandberg Networking the complete reference, third edition bobbi sandberg Networking the complete reference, third edition bobbi sandberg Networking the complete reference, third edition bobbi sandberg Networking the complete reference, third edition bobbi sandberg Networking the complete reference, third edition bobbi sandberg Networking the complete reference, third edition bobbi sandberg

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Great thanks and humble appreciation toall of those who helped with this book.And to my kids and their kids, and everand always to Sandy.

About the Author

Bobbi Sandberg is a small business consultant and retired CPA who has been a trainer, instructor,

and teacher of all things computer in the Pacific Northwest for more than 40 years She has “played”with computers since they occupied entire rooms and required perforated paper tape and punch cards.Today, she teaches hardware and software classes, solves hardware and software issues for a

number of clients, and keeps networks functional on a regular basis Bobbi is the author or coauthor

of several computer books, including QuickBooks 2015: The Small Business Guide, Quicken 2015:

The Official Guide, Quicken 2014: The Official Guide, Microsoft Office 2013 QuickSteps, and Computing for Seniors QuickSteps.

About the Technical Editors

Randal Nollan has been working with technology since the late 1970s when he wrote his first

program on pink punch cards Randal joined the U.S Navy in 1980 as an Aviation Ordnanceman andretired in 2001 During that time, he maintained the dBase III vaccination database for the squadroncorpsman and was always in the thick of maintaining the token ring network, computers, and terminalsthey had at the time He graduated from Skagit Valley College CIS (networking) and MIT

(programming) in 2003 He worked in Internet tech support from 2003 to 2005 and has since beenworking in computer repair for a local telephone company on Whidbey Island, Washington In hisspare time, he enjoys the outdoors by fishing, crabbing, bicycling, camping, and hunting Indoor funincludes playing with anything tech related, remodeling his home, and making wine from any fruit thatlands on his doorstep; some time he may even stop working and drink it

Dwight Spivey is the author of more than 20 books on computers and technology and has happily lent

his expertise as a technical editor to several more titles Dwight is happily married to Cindy, and theyreside on the Gulf Coast of Alabama along with their four children He studies theology, draws comicstrips, and roots for the Auburn Tigers in his ever-decreasing spare time

Van Aguirre is an information technology specialist who has broad experience in the field Since the

late 1990s, he has developed and taught courses in networking and multimedia technology, computingsecurity, computer crime forensics, IT risk management, IT business continuity, and disaster recoveryplanning Working with other IT professionals, he has planned and managed the implementation ofevolving technologies, including virtualization, mobile, and cloud computing to support institutionalbusiness and strategic initiatives As a project manager in educational technology, Van has

established and promoted successful apprenticeship programs in IT desk service management forcollege students, integrating LEAN principles and ITIL processes to supplement technical skills

AcknowledgmentsIntroduction

Part I Network Basics

Chapter 1 What Is a Network?

Local Area Network

Baseband vs BroadbandPacket Switching vs Circuit SwitchingCables and Topologies

Media Access ControlAddressing

Repeaters, Bridges, Switches, and RoutersWide Area Networks

Protocols and StandardsClients and ServersOperating Systems and Applications

Chapter 2 The OSI Reference Model

Communications Between the Layers

Data EncapsulationHorizontal CommunicationsVertical CommunicationsEncapsulation TerminologyThe Physical Layer

Physical Layer SpecificationsPhysical Layer SignalingThe Data Link Layer

AddressingMedia Access ControlProtocol IndicatorError DetectionThe Network Layer

RoutingFragmentingConnection-Oriented and Connectionless ProtocolsThe Transport Layer

Protocol Service Combinations

Transport Layer Protocol FunctionsSegmentation and ReassemblyFlow Control

Error Detection and RecoveryThe Session Layer

Dialog ControlDialog SeparationThe Presentation LayerThe Application Layer

Part II Network Hardware

Chapter 3 Network Interface Adapters

NIC FunctionsNIC Features

Full DuplexBus MasteringParallel TaskingWake-on-LAN or Wake-on-Wireless-LANSelecting a NIC

ProtocolTransmission SpeedNetwork InterfaceBus InterfaceBottlenecksISA or PCI?

Integrated AdaptersFiber-Optic NICsPortable SystemsHardware Resource Requirements

Power RequirementsServer vs Workstation NICs

Chapter 4 Network Interface Adapters and Connection Devices

RepeatersHubs

Passive HubsRepeating, Active, and Intelligent HubsToken Ring MAUs

Hub Configurations

The Uplink PortStackable HubsModular Hubs

Transparent BridgingBridge Loops

Source Route BridgingBridging Ethernet and Token Ring NetworksRouters

Router ApplicationsRouter FunctionsRouting TablesWindows Routing TablesRouting Table ParsingStatic and Dynamic RoutingSelecting the Most Efficient RouteDiscarding Packets

Packet FragmentationRouting and ICMPRouting ProtocolsSwitches

Switch TypesRouting vs SwitchingVirtual LANs

Layer 3 SwitchingMultiple-Layer Switching

Chapter 5 Cabling a Network

Cable Properties

Cabling StandardsData Link Layer Protocol StandardsCoaxial Cable

Thick EthernetThin EthernetCable TelevisionTwisted-Pair Cable

Unshielded Twisted-PairCategory 5e

Cat 6 and 6aCat 7

Connector PinoutsShielded Twisted-PairFiber-Optic Cable

Fiber-Optic Cable ConstructionFiber-Optic Connectors

Chapter 6 Wireless LANs

Wireless Networks

Advantages and Disadvantages of Wireless NetworksTypes of Wireless Networks

Wireless ApplicationsThe IEEE 802.11 Standards

The Physical LayerPhysical Layer FramesThe Data Link LayerData Link Layer FramesMedia Access Control

Chapter 7 Wide Area Networks

Introduction to TelecommunicationsWAN Utilization

Selecting a WAN TechnologyPSTN (POTS) ConnectionsLeased Lines

Leased-Line TypesLeased-Line HardwareLeased-Line ApplicationsISDN

ISDN ServicesISDN CommunicationsISDN Hardware

DSLSwitching Services

Packet-Switching ServicesCircuit-Switching ServicesFrame Relay

Frame-Relay HardwareVirtual Circuits

Frame-Relay MessagingATM

The Physical LayerThe ATM LayerThe ATM Adaptation LayerATM Support

SONET

Chapter 8 Server Technologies

Purchasing a ServerUsing Multiple Processors

Parallel ProcessingServer ClusteringUsing Hierarchical Storage ManagementFibre Channel Networking

Network Storage Subsystems

Chapter 9 Designing a Network

Reasoning the NeedSeeking ApprovalDesigning a Home or Small-Office Network

Selecting ComputersSelecting a Networking ProtocolChoosing a Network MediumChoosing a Network SpeedDesigning an Internetwork

Segments and BackbonesDistributed and Collapsed BackbonesBackbone Fault Tolerance

Selecting a Backbone LAN ProtocolConnecting to Remote NetworksSelecting a WAN TopologyPlanning Internet AccessLocating EquipmentWiring ClosetsData CentersFinalizing the Design

Part III Network Protocols

Chapter 10 Ethernet Basics

Ethernet DefinedEthernet Standards

Ethernet IIIEEE 802.3DIX Ethernet and IEEE 802.3 DifferencesIEEE Shorthand Identifiers

CSMA/CD

CollisionsLate CollisionsPhysical Layer Guidelines

10Base-5 (Thick Ethernet)10Base-2 (Thin Ethernet)10Base-T or 100Base-T (Twisted-Pair Ethernet)

Fiber-Optic EthernetCabling GuidelinesExceeding Ethernet Cabling SpecificationsThe Ethernet Frame

The IEEE 802.3 FrameThe Ethernet II FrameThe Logical Link Control SublayerThe SNAP Header

Full-Duplex Ethernet

Full-Duplex RequirementsFull-Duplex Flow ControlFull-Duplex Applications

Chapter 11 100Base Ethernet and Gigabit Ethernet

100Base Ethernet

Physical Layer OptionsCable Length RestrictionsAutonegotiation

The Logical Link Control SublayerThe MAC and RMAC SublayersThe Physical Medium–Independent SublayerThe Medium-Independent Interface SublayerThe Physical Medium–Dependent SublayerThe Medium-Dependent Interface

Working with 100VG-AnyLAN

Chapter 12 Networking Protocols

FDDI Topology

Part IV Network Systems

Chapter 13 TCP/IP

TCP/IP AttributesTCP/IP Architecture

The TCP/IP Protocol Stack

IP VersionsIPv4 AddressingSubnet Masking

IP Address RegistrationSpecial IP AddressesSubnetting

Ports and SocketsTCP/IP NamingTCP/IP Protocols

SLIP and PPPARP

IP

Chapter 14 Other TCP/IP Protocols

IPv6

IPv6 AddressesIPv6 Address StructureOther Protocols

ICMPUDPTCP

Chapter 15 The Domain Name System

Host Tables

Host Table ProblemsDNS ObjectivesDomain Naming

Top-Level DomainsSecond-Level DomainsSubdomains

DNS Functions

Resource RecordsDNS Name ResolutionReverse Name ResolutionDNS Name RegistrationZone Transfers

DNS Messaging

The DNS Header Section

The DNS Question SectionDNS Resource Record SectionsDNS Message Notation

Name Resolution MessagesRoot Name Server DiscoveryZone Transfer Messages

Chapter 16 Internet Services

Web Servers

Selecting a Web ServerHTML

HTTPFTP ServersFTP CommandsFTP Reply CodesFTP MessagingE-mail

E-mail AddressingE-mail Clients and ServersSimple Mail Transfer ProtocolPost Office Protocol

Internet Message Access Protocol

Part V Network Operating Services

Chapter 17 Windows

The Role of WindowsVersions

Service PacksMicrosoft Technical SupportOperating System Overview

Kernel Mode ComponentsUser Mode ComponentsServices

The Windows Networking Architecture

The NDIS InterfaceThe Transport Driver InterfaceThe Workstation Service

The Server ServiceAPIs

File Systems

FAT16FAT32

NTFSResilient File SystemThe Windows RegistryOptional Windows Networking Services

Active DirectoryMicrosoft DHCP ServerMicrosoft DNS ServerWindows Internet Naming Service

Chapter 18 Active Directory

Active Directory Architecture

Object TypesObject NamingDomains, Trees, and ForestsDNS and Active DirectoryGlobal Catalog ServerDeploying Active Directory

Creating Domain ControllersDirectory Replication

SitesMicrosoft Management ConsoleDesigning an Active Directory

Planning Domains, Trees, and Forests

Chapter 19 Linux

Understanding Linux

Linux DistributionsAdvantages and Disadvantages of LinuxFile Systems

Linux Installation QuestionsDirectory Structure

Quick Commands in LinuxWorking with Linux Files

JournalingEditingLack of Fragmentation

Chapter 20 Unix

Unix PrinciplesUnix ArchitectureUnix Versions

Unix System VBSD UnixUnix Networking

Using Remote Commands

Berkeley Remote CommandsDARPA Commands

Network File SystemClient-Server Networking

Chapter 21 Other Network Operating Systems and Networking in the Cloud

Historical Systems

FreeBSDNetBSDOpenBSDOracle SolarisOperating in the Cloud

History of the CloudBenefits of the CloudDisadvantages in the CloudHow the Cloud WorksCloud Types

Cloud Service Models

Infrastructure as a ServicePlatform as a ServiceSoftware as a ServiceNetwork as a Service

Part VI Network Services

Chapter 22 Network Clients

Windows Network Clients

Windows Networking ArchitectureNetWare Clients

Macintosh Clients

Connecting Macintosh Systems to Windows NetworksUnix Clients

ApplicationsUnix AccessWindows 7 InterfaceWindows 8 Interface

Chapter 23 Network Security Basics

Securing the File System

The Windows Security ModelWindows File System PermissionsUnix File System PermissionsVerifying Identities

FTP User AuthenticationKerberos

Public Key InfrastructureDigital Certificates

Token-Based and Biometric AuthenticationSecuring Network Communications

IPsecSSLFirewalls

Packet FiltersNetwork Address TranslationProxy Servers

Circuit-Level GatewaysCombining Firewall Technologies

Chapter 24 Wireless Security

Unsecured Home NetworksWireless Invasion ToolsUnderstanding Encryption

Chapter 25 Overview of Network Administration

Locating Applications and Data in Windows Systems

Server-Based Operating SystemsServer-Based ApplicationsStoring Data Files

Controlling the Workstation Environment

Drive Mappings in WindowsUser Profiles

Controlling the Workstation Registry

Using System Policies

Chapter 26 Network Management and Troubleshooting Tools

Operating System Utilities

Windows UtilitiesTCP/IP UtilitiesNetwork AnalyzersFiltering DataTraffic AnalysisProtocol AnalysisCable Testers

Selecting Backup TargetsBacking Up Open FilesRecovering from a DisasterJob Scheduling

Rotating MediaBackup Administration

Event LoggingPerforming Restores

Index

Acknowledgments

his book, like most others, is the end product of a lot of hard work by many people All of thepeople involved deserve great thanks A special thank-you to the following:

• Roger Stewart, acquisitions editor at McGraw-Hill Education, for his support,

understanding, and always available ear He and his team are unbeatable

• Two other members of the team, Patty Mon and Amanda Russell Patty is the finest editorialsupervisor around She is beyond helpful, always considerate and thoughtful, and just “there”for any questions She is a gem The generous, organized, and always on “top” of any concern

or issue, editorial coordinator Amanda Russell Amanda either has the answer at hand orfinds out quickly and reliably These few descriptive words are only the tip of the icebergwhen discussing their talent, professionalism, and always generous spirits

• The technical editors, Randy Nollan and Dwight Spivey, for the support, suggestions, andideas These skilled and proficient gentlemen made the process fun And a special thank-you

to Van Aguirre for his hard work at the beginning of the project

• Asheesh Ratra and his team at MPS Limited, who deserve great thanks and appreciation fortheir hard work and expertise It was a pleasure and honor working with them!

Introduction

his book is designed as a thorough, practical planning guide and underpinning of knowledge for ITnetworking professionals around the world, including students of IT networking courses,

beginning network administrators, and those seeking work in the IT networking field

Benefit to You, the Reader

After reading this book, you will be able to set up an effective network The book teaches everything,including methodology, analysis, case examples, tips, and all the technical supporting details needed

to suit an IT audience’s requirements, so it will benefit everyone from beginners to those who areintermediate-level practitioners

What This Book Covers

This book covers the details as well as the big picture for networking, including both physical andvirtual networks It discusses how to evaluate the various networking options and explains how tomanage network security and troubleshooting

Organization

This book is logically organized into six parts Within each part, the chapters start with basic

concepts and procedures, most of which involve specific networking tasks, and then work their way

up to more advanced topics

It is not necessary to read this book from beginning to end Skip around as desired The followingsections summarize the book’s organization and contents

Part I: Network Basics

This part of the book introduces networking concepts and explains both the OSI and TCP/IP models

• Chapter 1: What Is a Network?

• Chapter 2: The OSI Reference Model

Part II: Network Hardware

This part of the book discusses the various hardware items used in a computer network It also

explains some basics when designing a network

• Chapter 3: Network Interface Adapters

• Chapter 4: Network Interface Adapters and Connection Devices

• Chapter 5: Cabling a Network

• Chapter 6: Wireless LANs

• Chapter 7: Wide Area Networks

• Chapter 8: Server Technologies

• Chapter 9: Designing a Network

Part III: Network Protocols

This part of the book explains the various rules and protocols for networks

• Chapter 10: Ethernet Basics

• Chapter 11: 100Base Ethernet and Gigabit Ethernet

• Chapter 12: Networking Protocols

Part IV: Network Systems

This part of the book discusses the various network operating systems

• Chapter 13: TCP/IP

• Chapter 14: Other TCP/IP Protocols

• Chapter 15: The Domain Name System

• Chapter 16: Internet Services

Part V: Network Operating Services

In this part of the book, you will learn a bit more about the basics of some of the other servicesavailable, including cloud networking In Chapter 23, you will learn some of the basics needed tosecure your network

• Chapter 17: Windows

• Chapter 18: Active Directory

• Chapter 19: Linux

• Chapter 20: Unix

• Chapter 21: Other Network Operating Systems and Networking in the Cloud

Part VI: Network Services

From clients to security to the all-important backup, this section covers some of the day-to-dayoperations in networking

• Chapter 22: Network Clients

• Chapter 23: Network Security Basics

• Chapter 24: Wireless Security

• Chapter 25: Overview of Network Administration

• Chapter 26: Network Management and Troubleshooting Tools

• Chapter 27: Backing Up

Conventions

All how-to books—especially computer books—have certain conventions for communicating

information Here’s a brief summary of the conventions used throughout this book

Keystrokes are the keys you must press to complete a task There are two kinds of keystrokes:

• Keyboard shortcuts Combinations of keys you press to complete a task more quickly For

example, the shortcut for “clicking” a Cancel button may be to press the Esc key When youare to press a key, you will see the name of the key in small caps, like this: ESC If you mustpress two or more keys simultaneously, they are separated with a hyphen, like this: CTRL-P

• Literal text Text you must type in exactly as it appears in the book Although this book

doesn’t contain many instances of literal text, there are a few Literal text to be typed is in

boldface type, like this: Type help at the prompt.

• Monospace font Text that you see at the command line It looks like this:

Nslookup – nameserver

PART

CHAPTER

1 What Is a Network?

At its core, a network is simply two (or more) connected computers Computers can be connectedwith cables or telephone lines, or they can connect wirelessly with radio waves, fiber-optic lines, oreven infrared signals When computers are able to communicate, they can work together in a variety

of ways: by sharing their resources with each other, by distributing the workload of a particular task,

or by exchanging messages Today, the most widely used network is the Internet This book examines

in detail how computers on a network communicate; what functions they perform; and how to go aboutbuilding, operating, and maintaining them

The original model for collaborative computing was to have a single large computer connected to

a series of terminals, each of which would service a different user This was called time sharing

because the computer divided its processor clock cycles among the terminals Using this arrangement,the terminals were simply communications devices; they accepted input from users through a

keyboard and sent it to the computer When the computer returned a result, the terminal displayed it on

a screen or printed it on paper These terminals were sometimes called dumb terminals because they

didn’t perform any calculations on their own The terminals communicated with the main computer,never with each other

As time passed and technology progressed, engineers began to connect computers so that theycould communicate At the same time, computers were becoming smaller and less expensive, givingrise to mini- and microcomputers The first computer networks used individual links, such as

telephone connections, to connect two systems There are a number of computer networking types andseveral methods of creating these types, which will be covered in this chapter

Local Area Network

Soon after the first IBM PCs hit the market in the 1980s and rapidly became accepted as a businesstool, the advantages of connecting these small computers became obvious Rather than supplyingevery computer with its own printer, a network of computers could share a single printer When oneuser needed to give a file to another user, a network eliminated the need to swap floppy disks Theproblem, however, was that connecting a dozen computers in an office with individual point-to-pointlinks between all of them was not practical The eventual solution to this problem was the local areanetwork (LAN)

A LAN is a group of computers connected by a shared medium, usually a cable By sharing asingle cable, each computer requires only one connection and can conceivably communicate with anyother computer on the network A LAN is limited to a local area by the electrical properties of thecables used to construct them and by the relatively small number of computers that can share a singlenetwork medium LANs are generally restricted to operation within a single building or, at most, acampus of adjacent buildings

Some technologies, such as fiber optics, have extended the range of LANs to several kilometers,but it isn’t possible to use a LAN to connect computers in distant cities, for example That is the

province of the wide area network (WAN), as discussed later in this chapter

In most cases, a LAN is a baseband, packet-switching network An understanding of the terms

baseband and packet switching, which are examined in the following sections, is necessary to

understand how data networks operate because these terms define how computers transmit data overthe network medium

Baseband vs Broadband

A baseband network is one in which the cable or other network medium can carry only a single signal

at any one time A broadband network, on the other hand, can carry multiple signals simultaneously,using a discrete part of the cable’s bandwidth for each signal As an example of a broadband

network, consider the cable television service you probably have in your home Although only onecable runs to your TV, it supplies you with dozens of channels of programming at the same time Ifyou have more than one television connected to the cable service, the installer probably used a

splitter (a coaxial fitting with one connector for the incoming signals and two connectors for outgoingsignals) to run the single cable entering your house to two different rooms The fact that the TVs can

be tuned to different programs at the same time while connected to the same cable proves that thecable is providing a separate signal for each channel at all times A baseband network uses pulsesapplied directly to the network medium to create a single signal that carries binary data in encodedform Compared to broadband technologies, baseband networks span relatively short distances

because they are subject to degradation caused by electrical interference and other factors The

effective maximum length of a baseband network cable segment diminishes as its transmission rateincreases This is why local area networking protocols such as Ethernet have strict guidelines forcable installations

NOTE A cable segment is an unbroken network cable that connects two

nodes.

Packet Switching vs Circuit Switching

LANs are called packet-switching networks because their computers divide their data into small,

discrete units called packets before transmitting it There is also a similar technique called cell

switching, which differs from packet switching only in that cells are always a consistent, uniform

size, whereas the size of packets is variable Most LAN technologies, such as Ethernet, Token Ring,and Fiber Distributed Data Interface (FDDI), use packet switching Asynchronous Transfer Mode(ATM) is the cell-switching LAN protocol that is most commonly used

Understanding Packets

E-mail may be the easiest way to understand packets Each message is divided by the sendingservice into a specific number of bytes, often between 1,000 and 1,500 Then each packet is sentusing the most efficient route For example, if you are sending an e-mail to your company’s homeoffice from your vacation cabin, each packet will probably travel along a different route This ismore efficient, and if any one piece of equipment is not working properly in the network while amessage is being transferred, the packet that would use that piece of equipment can be routed

around the problem area and sent on another route When the message reaches its destination, thepackets are reassembled for delivery of the entire message.

Segmenting the data in this way is necessary because the computers on a LAN share a single

cable, and a computer transmitting a single unbroken stream of data would monopolize the networkfor too long If you were to examine the data being transmitted over a packet-switching network, youwould see the packets generated by several different systems intermixed on the cable The receivingsystem, therefore, must have a mechanism for reassembling the packets into the correct order andrecognizing the absence of packets that may have been lost or damaged in transit

The opposite of packet switching is circuit switching, in which one system establishes a

dedicated communication channel to another system before any data is transmitted In the data

networking industry, circuit switching is used for certain types of wide area networking technologies,such as Integrated Services Digital Network (ISDN) and frame relay The classic example of a

circuit-switching network is the public telephone system When you place a call to another person, aphysical circuit is established between your telephone and theirs This circuit remains active for theentire duration of the call, and no one else can use it, even when it is not carrying any data (that is,when no one is talking)

In the early days of the telephone system, every phone was connected to a central office with adedicated cable, and operators using switchboards manually connected a circuit between the twophones for every call While today the process is automated and the telephone system transmits manysignals over a single cable, the underlying principle is the same

LANs were originally designed to connect a small number of computers into what later came to

be called a workgroup Rather than investing a huge amount of money into a large, mainframe

computer and the support system needed to run it, business owners came to realize that they couldpurchase a few computers, cable them together, and perform most of the computing tasks they needed

As the capabilities of personal computers and applications grew, so did the networks, and the

technology used to build them progressed as well

Cables and Topologies

Most LANs are built around copper cables that use standard electrical currents to relay their signals.Originally, most LANs consisted of computers connected with coaxial cables, but eventually, thetwisted-pair cabling used for telephone systems became more popular Another alternative is fiber-optic cable, which doesn’t use electrical signals at all but instead uses pulses of light to encode

binary data Other types of network infrastructures eliminate cables entirely and transmit signals usingwhat is known as unbounded media, such as radio waves, infrared, and microwaves

NOTE For more information about the various types of cables used in

data networking, see Chapter 5

LANs connect computers using various types of cabling patterns called topologies (see Figure

1-1), which depend on the type of cable used and the protocols running on the computers The mostcommon topologies are as follows:

• Bus A bus topology takes the form of a cable that runs from one computer to the next one in a

daisy-chain fashion, much like a string of Christmas tree lights All of the signals transmitted

by the computers on the network travel along the bus in both directions to all of the othercomputers The two ends of the bus must be terminated with electrical resistors that nullifythe voltages reaching them so that the signals do not reflect in the other direction The

primary drawback of the bus topology is that, like the string of Christmas lights it resembles,

a fault in the cable anywhere along its length splits the network in two and prevents systems

on opposite sides of the break from communicating In addition, the lack of termination ateither half can prevent computers that are still connected from communicating properly Aswith Christmas lights, finding a single faulty connection in a large bus network can be

troublesome and time consuming Most coaxial cable networks, such as the original EthernetLANs, use a bus topology

• Star (hub and spoke) A star topology uses a separate cable for each computer that runs to a

central cabling nexus called a hub or concentrator The hub propagates the signals entering

through any one of its ports out through all of the other ports so that the signals transmitted byeach computer reach all the other computers Hubs also amplify the signals as they processthem, enabling them to travel longer distances without degrading A star network is morefault tolerant than a bus because a break in a cable affects only the device to which that cable

is connected, not the entire network Most of the networking protocols that call for pair cable, such as 10Base-T and 100Base-T Ethernet, use the star topology

twisted-• Star bus A star bus topology is one method for expanding the size of a LAN beyond a single

star In this topology, a number of star networks are joined together using a separate bus

cable segment to connect their hubs Each computer can still communicate with any othercomputer on the network because each of the hubs transmits its incoming traffic out throughthe bus port as well as the other star ports Designed to expand 10Base-T Ethernet networks,the star bus is rarely seen today because of the speed limitations of coaxial bus networks,which can function as a bottleneck that degrades the performance of faster star network

technologies such as Fast Ethernet

• Ring This topology is similar to a bus topology, except these topologies transmit in one

direction only from station to station A ring topology often uses separate physical ports andwires to send and receive data A ring topology is functionally equivalent to a bus topologywith the two ends connected so that signals travel from one computer to the next in an endlesscircular fashion However, the communications ring is only a logical construct, not a physicalone The physical network is actually cabled using a star topology, and a special hub called a

multistation access unit (MSAU) implements the logical ring by taking each incoming signal

and transmitting it out through the next downstream port only (instead of through all of theother ports, like a star hub) Each computer, upon receiving an incoming signal, processes it(if necessary) and sends it right back to the hub for transmission to the next station on the ring.Because of this arrangement, systems that transmit signals onto the network must also removethe signals after they have traversed the entire ring Networks configured in a ring topologycan use several different types of cable Token Ring networks, for example, use twisted-paircables, while FDDI networks use the ring topology with fiber-optic cable

• Daisy chains These topologies are the simplest form as one device is connected to another

through serial ports Think of a computer hooked to a printer and the printer, in turn, beinghooked to a laptop

• Hierarchical star The hierarchical star topology is the most common method for expanding a

star network beyond the capacity of its original hub When a hub’s ports are all filled and youhave more computers to connect to the network, you can connect the original hub to a secondhub using a cable plugged into a special port designated for this purpose Traffic arriving ateither hub is then propagated to the other hub as well as to the connected computers Thenumber of hubs that a single LAN can support is dependent on the protocol it uses

Figure 1-1 Common cable topographies

The topologies discussed here are physical topologies, which differ from logical topologies that

are discussed in later chapters Physical topologies refer to the placement of cables and other

components of the network Logical topologies refer to the flow of data on the network

Media Access Control

When multiple computers are connected to the same baseband network medium, there must be a

media access control (MAC) mechanism that arbitrates access to the network to prevent systems fromtransmitting data at the same time A MAC mechanism is a fundamental part of all local area

networking protocols that use a shared network medium The two most common MAC mechanismsare Carrier Sense Multiple Access with Collision Detection (CSMA/CD), which is used by Ethernetnetworks, and token passing, which is used by Token Ring, FDDI, and other protocols These twomechanisms are fundamentally different, but they accomplish the same task by providing each system

on the network with an equal opportunity to transmit its data (For more information about these MACmechanisms, see Chapter 10 for CSMA/CD and Chapter 12 for token passing.)

Addressing

For systems on a shared network medium to communicate effectively, they must have some means ofidentifying each other, usually some form of numerical address In most cases, the network interfacecard (NIC) installed into each computer has an address hard-coded into it at the factory, called its

MAC address or hardware address, which uniquely identifies that card among all others Every

packet that each computer transmits over the network contains the address of the sending computerand the address of the system for which the packet is intended

In addition to the MAC address, systems may have other addresses operating at other layers Forexample, Transmission Control Protocol/Internet Protocol (TCP/IP) requires that each system beassigned a unique IP address in addition to the MAC address it already possesses Systems use thevarious addresses for different types of communications (See Chapter 3 for more information onMAC addressing and Chapter 13 for more information on IP addressing.)

Repeaters, Bridges, Switches, and Routers

LANs were originally designed to support only a relatively small number of computers—30 for thinEthernet networks and 100 for thick Ethernet—but the needs of businesses quickly outgrew theselimitations To support larger installations, engineers developed products that enabled administrators

to connect two or more LANs into what is known as an internetwork, which is essentially a network

of networks that enables the computers on one network to communicate with those on another Don’t

confuse the generic term internetwork with the Internet The Internet is an example of an extremely

large internetwork, but any installation that consists of two or more LANs connected is also an

internetwork This terminology is confusing because it is so often misused Sometimes what usersmean when they refer to a network is actually an internetwork, and at other times, what may seem to

be an internetwork is actually a single LAN Strictly speaking, a LAN or a network segment is a

group of computers that share a network cable so that a broadcast message transmitted by one systemreaches all of the other systems, even if that segment is actually composed of many pieces of cable.For example, on a typical 10Base-T Ethernet LAN, all of the computers are connected to a hub usingindividual lengths of cable Regardless of that fact, this arrangement is still an example of a networksegment or LAN Individual LANs can be connected using several different types of devices, some ofwhich simply extend the LAN while another creates an internetwork These devices are as follows:

• Repeaters A repeater is a purely electrical device that extends the maximum distance a LAN

cable can span by amplifying the signals passing through it The hubs used on star networks

are sometimes called multiport repeaters because they have signal amplification capabilities

integrated into the unit Stand-alone repeaters are also available for use on coaxial networks

to extend them over longer distances Using a repeater to expand a network segment does notdivide it into two LANs or create an internetwork

• Bridges A bridge provides the amplification function of a repeater, along with the ability to

selectively filter packets based on their addresses Packets that originate on one side of thebridge are propagated to the other side only if they are addressed to a system that exists there.Because bridges do not prevent broadcast messages from being propagated across the

connected cable segments, they, too, do not create multiple LANs or transform a network into

an internetwork

• Switches Switches are revolutionary devices that in many cases eliminate the shared network

medium entirely A switch is essentially a multiport repeater, like a hub, except that instead

of operating at a purely electrical level, the switch reads the destination address in each

incoming packet and transmits it out only through the port to which the destination system isconnected

• Routers A router is a device that connects two LANs to form an internetwork Like a bridge,

a router forwards only the traffic that is destined for the connected segment, but unlike

repeaters and bridges, routers do not forward broadcast messages Routers can also connectdifferent types of networks (such as Ethernet and Token Ring), whereas bridges and repeaterscan connect only segments of the same type

Wide Area Networks

Internetworking enables an organization to build a network infrastructure of almost unlimited size Inaddition to connecting multiple LANs in the same building or campus, an internetwork can connectLANs at distant locations through the use of wide area network links A WAN is a collection of

LANs, some or all of which are connected using point-to-point links that span relatively long

distances A typical WAN connection consists of two routers, one at each LAN site, connected using

a long-distance link such as a leased telephone line Any computer on one of the LANs can

communicate with the other LAN by directing its traffic to the local router, which relays it over theWAN link to the other site

WAN links differ from LANs in that they do not use a shared network medium and they can spanmuch longer distances Because the link connects only two systems, there is no need for media accesscontrol or a shared network medium An organization with offices located throughout the world canbuild an internetwork that provides users with instantaneous access to network resources at any

location The WAN links themselves can use technologies ranging from telephone lines to public datanetworks to satellite systems Unlike a LAN, which is nearly always privately owned and operated,

an outside service provider (such as a telephone company) is nearly always involved in a WAN

connection because private organizations don’t usually own the technologies needed to carry signalsover such long distances Generally speaking, WAN connections can be slower and more expensivethan LANs, and sometimes much more so As a result, one of the goals of the network administrator is

to maximize the efficiency of WAN traffic by eliminating unnecessary communications and choosingthe best type of link for the application See Chapter 7 for more information on WAN technologies

There are also wireless LAN/WAN networks and metropolitan area networks (MANs) A MAN

has three features that differentiate it from both a LAN and a WAN:

• A MAN’s size is usually between that of a LAN and a WAN Typically, it covers between 3and 30 miles (5 to 50 km) A MAN can encompass several buildings, a company campus, or

a small town

• As with WANs, MANs are normally owned by a group or a network provider

• MANs are often used as a way to provide shared access to one or more WANs

Protocols and Standards

Communications between computers on a network are defined by protocols, standardized methodsthat the software programs on the computers have in common These protocols define every part ofthe communications process, from the signals transmitted over network cables to the query languagesthat enable applications on different machines to exchange messages Networked computers run a

series of protocols, called a protocol stack, that spans from the application user interface at the top to

the physical network interface at the bottom The stack is traditionally split into seven layers TheOpen Systems Interconnection (OSI) reference model defines the functions of each layer and how thelayers work together to provide network communications Chapter 2 covers the OSI reference model

• Institute of Electrical and Electronic Engineers (IEEE) A U.S.-based society responsible

for the publication of the IEEE 802 working group, which includes the standards that definethe protocols commonly known as Ethernet and Token Ring, as well as many others

• International Organization for Standardization (ISO) A worldwide federation of

standards bodies from more than 100 countries, responsible for the publication of the OSIreference model document

• Internet Engineering Task Force (IETF) An ad hoc group of contributors and consultants

who collaborate to develop and publish standards for Internet technologies, including theTCP/IP protocols

Clients and Servers

Local area networking is based on the client-server principle, in which the processes needed to

accomplish a particular task are divided between computers functioning as clients and servers This

is in direct contrast to the mainframe model, in which the central computer did all of the processingand simply transmitted the results to a user at a remote terminal A server is a computer running aprocess that provides a service to other computers when they request it A client is the computer

running a program that requests the service from a server

For example, a LAN-based database application stores its data on a server, which stands by,waiting for clients to request information from it Users at workstation computers run a database

client program in which they generate queries that request specific information in the database andtransmit those queries to the server The server responds to the queries with the requested informationand transmits it to the workstations, which format it for display to the users In this case, the

workstations are responsible for providing a user interface and translating the user input into a querylanguage understood by the server They are also responsible for taking the raw data from the serverand displaying it in a comprehensible form to the user The server may have to service dozens orhundreds of clients, so it is still a powerful computer By offloading some of the application’s

functions to the workstations, however, its processing burden is nowhere near what it would be on amainframe system

Operating Systems and Applications

Clients and servers are actually software components, although some people associate them withspecific hardware elements This confusion is because some network operating systems require that acomputer be dedicated to the role of server and that other computers function solely as clients This is

a client-server operating system, as opposed to a peer-to-peer operating system, in which every

computer can function as both a client and a server The most basic client-server functionality

provided by a network operating system (NOS) is the ability to share file system drives and printers,and this is what usually defines the client and server roles At its core, a NOS makes services

available to its network clients The system can provide the following:

• Printer services, including managing devices, print jobs, who is using what asset, and whatassets are not available to the network

• Managing user access to files and other resources, such as the Internet

• System monitoring, including providing network security

• Making network administration utilities available to network administrators

Apart from the internal functions of network operating systems, many LAN applications and

network services also operate using the client-server paradigm Internet applications, such as theWorld Wide Web, consist of servers and clients, as do administrative services such as the DomainName System (DNS)

Most of today’s desktop operating systems are capable of providing some of the services

traditionally ascribed to NOSs since many small-office/home-office (SOHO) LAN implementationstake advantage of the fact Understanding this may help clarify the distinction between LANs that aretruly client-server, relying on network operating systems, and those network configurations that

leverage powerful computers with today’s operating systems These operating systems are not limited

to computers, but can include cell phones, tablets, and other products that are not considered to be

“computers.”

CHAPTER

2 The OSI Reference Model

Network communications take place on many levels and can be difficult to understand, even for theknowledgeable network administrator The Open Systems Interconnection (OSI) reference model is atheoretical construction that separates network communications into seven distinct layers, as shown in

Figure 2-1 Each computer on the network uses a series of protocols to perform the functions assigned

to each layer The layers collectively form what is known as the protocol stack or networking stack.

At the top of the stack is the application that makes a request for a resource located elsewhere on thenetwork, and at the bottom is the physical medium that actually connects the computers and forms thenetwork, such as a cable

Figure 2-1 The OSI reference model with its seven layers

The OSI reference model was developed in two separate projects by the International

Organization for Standardization (ISO) and the Comité Consultatif International Téléphonique et

Télégraphique (Consultative Committee for International Telephone and Telegraphy, or CCITT),which is now known as the Telecommunications Standardization Sector of the International

Telecommunications Union (ITU-T) Each of these two bodies developed its own seven-layer model,but the two projects were combined in 1983, resulting in a document called “The Basic ReferenceModel for Open Systems Interconnection” that was published by the ISO as ISO 7498 and by the ITU-

T as X.200

The OSI stack was originally conceived as the model for the creation of a protocol suite thatwould conform exactly to the seven layers This suite never materialized in a commercial form,

however, and the model has since been used as a teaching, reference, and communications tool

Networking professionals, educators, and authors frequently refer to protocols, devices, or

applications as operating at a particular layer of the OSI model because using this model breaks acomplex process into manageable units that provide a common frame of reference Many of the

chapters in this book use the layers of the model to help define networking concepts However, it isimportant to understand that none of the protocol stacks in common use today conforms exactly to thelayers of the OSI model In many cases, protocols have functions that overlap two or more layers,such as Ethernet, which is considered a data link layer protocol but which also defines elements of

the physical layer.

The primary reason why real protocol stacks differ from the OSI model is that many of the

protocols used today (including Ethernet) were conceived before the OSI model documents werepublished In fact, the TCP/IP protocols have their own layered model, which is similar to the OSImodel in several ways but uses only four layers (see Figure 2-2) In addition, developers are usuallymore concerned with practical functionality than with conforming to a preexisting model The seven-layer model was designed to separate the functions of the protocol stack in such a way as to make itpossible for separate development teams to work on the individual layers, thus streamlining the

development process However, if a single protocol can easily provide the functions that are defined

as belonging in separate layers of the model, why divide it into two separate protocols just for thesake of conformity?

Figure 2-2 The OSI reference model and the TCP/IP protocol stack

Communications Between the Layers

Networking is the process of sending messages from one place to another, and the protocol stackillustrated in the OSI model defines the basic components needed to transmit messages to their

destinations The communication process is complex because the applications that generate the

messages have varying requirements Some message exchanges consist of brief requests and repliesthat have to be exchanged as quickly as possible and with a minimum amount of overhead Other

network transactions, such as program file transfers, involve the transmission of larger amounts ofdata that must reach the destination in perfect condition, without alteration of a single bit Still othertransmissions, such as streaming audio or video, consist of huge amounts of data that can survive theloss of an occasional bit, byte, or packet, but that must reach the destination in a timely manner

The networking process also includes a number of conversions that ultimately take the applicationprogramming interface (API) calls generated by applications and transform them into electrical

charges, pulses of light, or other types of signals that can be transmitted across the network medium.Finally, the networking protocols must see to it that the transmissions reach the appropriate

destinations in a timely manner Just as you package a letter by placing it in an envelope and writing

an address on it, the networking protocols package the data generated by an application and address it

to another computer on the network

Data Encapsulation

To satisfy all of the requirements just described, the protocols operating at the various layers worktogether to supply a unified quality of service Each layer provides a service to the layers directlyabove and below it Outgoing traffic travels down through the stack to the network physical medium,acquiring the control information needed to make the trip to the destination system as it goes Thiscontrol information takes the form of headers (and in one case a footer) that surround the data

received from the layer above, in a process called data encapsulation The headers and footer arecomposed of individual fields that contain control information (necessary/required by the system todeliver) used to get the packet to its destination In a sense, the headers and footer form the envelopethat carries the message received from the layer above

In a typical transaction, shown in Figure 2-3, an application layer protocol (which also includespresentation and session layer functions) generates a message that is passed down to a transport layerprotocol The protocol at the transport layer has its own packet structure, called a protocol data unit(PDU), which includes specialized header fields and a data field that carries the payload In this case,the payload is the data received from the application layer protocol By packaging the data in its ownPDU, the transport layer encapsulates the application layer data and then passes it down to the nextlayer

Figure 2-3 The application layer data is encapsulated for transmission by the protocols at the lower layers in the stack.

The network layer protocol then receives the PDU from the transport layer and encapsulates itwithin its own PDU by adding a header and using the entire transport layer PDU (including the

application layer data) as its payload The same process occurs again when the network layer passes

its PDU to the data link layer protocol, which adds a header and footer To a data link layer protocol,the data within the frame is treated as payload only, just as postal employees have no idea what isinside the envelopes they process The only system that reads the information in the payload is thecomputer possessing the destination address That computer then either passes the network layerprotocol data contained in the payload up through its protocol stack or uses that data to determinewhat the next destination of the packet should be In the same way, the protocols operating at the otherlayers are conscious of their own header information but are unaware of what data is being carried inthe payload.

Once it is encapsulated by the data link layer protocol, the completed packet (now called a

frame) is then ready to be converted to the appropriate type of signal used by the network medium.Thus, the final packet, as transmitted over the network, consists of the original application layer dataplus several headers applied by the protocols at the succeeding layers, as shown in Figure 2-4

Figure 2-4 An encapsulated frame, ready for transmission

NOTE Each layer must translate data into its specific format before

sending it on Therefore, each layer creates its own PDU to

transmit to the next layer As each layer receives data, the PDU of the previous layer is read, and a new PDU is created using that

layer’s protocol Remember, a PDU is a complete message (or

packet) that includes the protocol of the sending layer At the

physical layer, you end up with a message that consists of all the data that has been encapsulated with the headers and/or footers

from each of the previous layers.

Horizontal Communications

For two computers to communicate over a network, the protocols used at each layer of the OSI model

in the transmitting system must be duplicated at the receiving system When the packet arrives at itsdestination, the process by which the headers are applied at the source is repeated in reverse Thepacket travels up through the protocol stack, and each successive header is stripped off by the

appropriate protocol and processed In essence, the protocols operating at the various layers

communicate horizontally with their counterparts in the other system, as shown in Figure 2-5

Figure 2-5 Each layer has logical connections with its counterpart in other systems.

The horizontal connections between the various layers are logical; there is no direct

communication between them The information included in each protocol header by the transmittingsystem is a message that is carried to the same protocol in the destination system

Vertical Communications

The headers applied by the various protocols implement the specific functions carried out by thoseprotocols In addition to communicating horizontally with the same protocol in the other system, theheader information enables each layer to communicate with the layers above and below it, as shown

in Figure 2-6 For example, when a system receives a packet and passes it up through the protocolstack, the data link layer protocol header includes a field that identifies which network layer protocolthe system should use to process the packet The network layer protocol header in turn specifies one

of the transport layer protocols, and the transport layer protocol identifies the application for whichthe data is ultimately destined This vertical communication makes it possible for a computer to

support multiple protocols at each of the layers simultaneously As long as a packet has the correctinformation in its headers, it can be routed on the appropriate path through the stack to the intendeddestination