Chapter 13, Troubleshooting a Network Chapter 13, Lesson 1 Understanding the Problem 1. Troubleshooting A. Overview 1. Most difficult task computer professionals face. 2. Computer never seems to fail at a convenient time. 3. Pressure to fix the problem immediately is intense. 4. It is essential to isolate the true cause of the problem from irrelevant factors. 5. After the problem has been diagnosed, locating resources and following the procedures required to correct the problem are straightforward. 6. More of an art form than an exact science. 7. Must approach the problem systematically. 8. Look for the cause, not the symptoms. 9. Focus on the things that might be the cause of the problem. 10. Eliminate as many alternative causes as possible. B. Step 1: Defining the Problem 1. First phase is the most critical, yet most often ignored. 2. Without a complete understanding of the entire problem, a great deal of time can be spent working on the symptoms, without getting to the cause. 3. Client or network user is the best source of information. 4. Daily users probably recall the events that led up to the failure. 5. List the sequence of events before the failure. 6. Ask users what the network is doing or not doing that makes them think it is not functioning correctly. 7. General questions to ask might include: a. When did you first notice the problem or error? b. Has the computer recently been moved? c. Have there been any recent software or hardware changes? d. Has anything happened to the workstation? Was it dropped or was something dropped on it? Was coffee or soda spilled on the keyboard? e. When exactly does the problem or error occur? During the startup process? After lunch? Only on Monday mornings? After sending an e-mail message? f. Can you reproduce the problem or error? If so, how do you reproduce the problem? g. What does the problem or error look like? h. Describe any changes in the computer (such as noises, screen changes, disk activity lights). 8. User observations can be clues to the network problem. a. “The network is really slow.” b. “I cannot connect to the server.” c. “I was connected to the server, but I lost the connection.” d. “One of my applications will not run.” e. “I cannot print.” C. Step 2: Isolating the Cause 1. Isolate the problem. 2. Eliminate the most obvious problems and work toward the more complex and obscure. 3. Narrow search down to one or two general categories. 4. Observe the failure first-hand. 5. Have someone demonstrate the failure. 6. If it is not an operator-induced problem, it is important to observe how it is created, as well as the results. 7. The most difficult problems to isolate are those that are intermittent. 8. Re-create the set of circumstances that cause the failure. 9. Sometimes eliminating causes that are not the problem is the best approach. 10. This process takes time and patience. 11. User needs to keep detailed records of what is being done before and when the failure occurs. 12. Ask the user to refrain from doing anything with the computer when the problem recurs, except to call for assistance. 13. Administrator should refer to documented baseline information to compare with current network behavior. 14. Rerun tests under the same set of conditions as prevailed when the baseline was created, then compare the two results. 15. Changes between the two can indicate the source of the problem. 16. Information gathering involves scanning the network and looking for an obvious cause of the problem. 17. Review the documented history of the network to determine if the problem has occurred before and, if so, whether there is a recorded solution. D. Step 3: Planning the Repair 1. Create a planned approach to isolating the problem based on obtained knowledge. 2. Follow the designed plan. 3. Jumping ahead and randomly trying things out of order can often lead to problems. 4. If the first plan is not successful, create a new plan based on what was discovered with the previous plan. 5. Reassess any assumptions made in the previous plan.

2 Outline, Chapter 13 Networking Essentials Plus, Third Edition 6. Once the problem is located, either repair the defect or replace the defective component. 7. If it is a software problem, be sure to record the “before” and “after” changes. E. Step 4: Confirming the Results 1. No repair is complete without confirmation that the job has been successfully concluded. 2. Make sure that the problem no longer exists. 3. Ask the user to test the solution and confirm the results. 4. Make sure the fix did not generate new problems. F. Step 5: Documenting the Outcome 1. Documenting the problem and repair produces invaluable information. 2. There is no substitute for experience in troubleshooting, and each new problem presents an opportunity to expand that experience. 3. Keeping a copy of the repair procedure in the technical library can be useful if the problem occurs again. 4. Changes made can affect the baseline. 5. Might need to update the network baseline in anticipation of future problems and needs. 2. Segmenting the Problem A. Overview 1. Divide the network into smaller parts to isolate the cause of the problem. 2. Isolating or removing a portion of the network will help to get the rest of the network up and operational again. 3. If removing a portion solved the problem for the rest of the network, the search for the problem can be focused on the part that was removed. 4. Ask whether the problem stems from the hardware or the software. B. Hardware 1. If it is a hardware problem, start by looking at only one segment of the network, then at only one type of hardware. 2. When assessing hardware performance problems, use the information obtained from the hardware baselines to compare against the current symptoms and performance. 3. Check hardware and network components including: a. NICs b. Cabling and connectors c. Clients/workstations d. Connectivity components such as repeaters, bridges, routers, brouters, and gateways e. Hubs f. Protocols g. Servers h. Users C. Protocols

Outline, Chapter 13 3 Networking Essentials Plus, Third Edition 1. Network protocols require special attention because they are designed to bypass network problems and attempt to overcome network faults. 2. Most protocols use “retry logic,” in which the software attempts an automatic recovery from a problem. 3. This results in slow network performance. 4. Failing hardware devices, such as hard drives and controllers, will use retry logic by repeatedly interrupting the CPU for more processing time to complete their task. 3. Isolating the Problem A. Rank the list of possible causes in order from the most likely to least likely cause of the problem. B. Select the most likely candidate from the list of possible causes, test it, and see if that is the problem. C. Start from the most obvious and work to the most difficult. 4. Setting Priorities A. A fundamental element in network problem solving. B. Everyone wants his or her computer fixed first, so setting priorities is not an easy job. C. Simplest approach is to prioritize on a “first come, first served” basis. D. Some failures are more critical to resolve than others. E. Assess the problem’s impact on the ability to maintain operations. Chapter 13, Lesson 2 Troubleshooting Tools 1. Hardware Tools A. Overview 1. Once were very expensive and difficult devices to use, but today they are less expensive and easier to use. 2. Helpful in identifying performance trends and problems. B. Digital Voltmeters 1. Also called volt-ohm meter. 2. Primary all-purpose electronic measuring tool. 3. Standard equipment for any computer or electronic technician. 4. Can reveal far more than just the amount of voltage passing through resistance. 5. Network administrator’s most important function is to confirm source voltage for the network equipment. 6. Low voltage often causes intermittent faults. 7. Voltage that is too high can cause immediate damage to the equipment. 8. Voltmeters can determine if: a. Cable is continuous (has no breaks). b. Cable can carry network traffic.

4 Outline, Chapter 13 Networking Essentials Plus, Third Edition c. Two parts of the same cable are exposed and touching (thereby causing shorts). d. Exposed cable is touching another conductor, such as a metal surface. C. Time Domain Reflectometers (TDRs) 1. Send sound pulses similar to sonar along cables to locate breaks, shorts, or imperfections. 2. Network performance suffers when the cable is not intact. 3. Problem is analyzed and the results are displayed. 4. Can locate a break within a few feet of the actual separation in the cable. 5. Used heavily during the installation of a new network and invaluable in troubleshooting and maintaining existing networks. 6. Requires special training, and not every maintenance department will have this equipment. D. Advanced Cable Testers 1. Work beyond the physical layer of the OSI model in the data-link layer, network layer, and even the transport layer. 2. Also display information about the condition of the physical cable. E. Other Hardware Tools 1. Crossover Cables a. Crossover cables are used to connect two computers directly with a single patch cable. b. Send and receive wires are reversed on one end; the send wire from one computer is connected to the receive port on the other computer. c. Useful in troubleshooting network connection problems. d. Two computers can be directly connected, bypassing the network and making it possible to isolate and test the communication capabilities of one computer, rather than the whole network. 2. Hardware Loopback a. Is a serial port connector that tests the communication capabilities of a computer’s serial port without having to connect to another computer or peripheral device. b. Data is transmitted to a line, then returned as received data. c. If the transmitted data does not return, the hardware loopback detects a hardware malfunction. 3. Tone Generator and Tone Locator a. Tone generators are standard tools for wiring technicians in all fields. b. Used to apply an alternating or continuous tone signal to a cable or a conductor. c. Attached to one end of the cable in question. d. A matching tone locator is used to detect the correct cable at the other end of the run. e. Able to test for wiring continuity and line polarity. f. Used to trace twisted-pair wiring, single conductors, and coaxial cables, among others. g. This pair is sometimes referred to as “fox and hound.”

Outline, Chapter 13 5 Networking Essentials Plus, Third Edition 4. Oscilloscopes a. Electronic instruments that measure the amount of signal voltage per unit of time and display the result on a monitor. b. When used with TDRs, an oscilloscope can display: (1) Shorts (2) Sharp bends or crimps in the cable (3) Opens (breaks in the cable) (4) Attenuation (loss of signal power) 2. Software Tools A. Network Monitors 1. Software tools that track all or a selected part of network traffic. 2. Examine data packets and gather information about packet types, errors, and packet traffic to and from each computer. 3. Useful for establishing part of the network baseline. 4. After the baseline has been established, it is possible to troubleshoot traffic problems and monitor network usage to determine when it is time to upgrade. B. Protocol Analyzers 1. Also called network analyzers; perform real-time network traffic analysis using packet capture, decoding, and transmission data. 2. Network administrators who work with large networks rely heavily on the protocol analyzer. 3. Used most often to monitor network interactivity. 4. Look inside the packet to identify a problem. 5. Have built-in TDRs. 6. Generate statistics based on network traffic to help create a picture of the network, including: a. Cabling b. Software c. File servers d. Workstations e. NICs 7. Provide insights and detect network problems including: a. Faulty network components b. Configuration or connection errors c. LAN bottlenecks d. Traffic fluctuations e. Protocol problems f. Conflicting applications g. Unusual server traffic 8. Identify a wide range of network behavior, such as: a. The most active computers. b. Computers that are sending error-filled packets.

6 Outline, Chapter 13 Networking Essentials Plus, Third Edition c. View and filter certain types of packets that are helpful for routing traffic, and can determine what type of traffic is passing across a given network segment. d. Track network performance to identify trends that help the administrator better plan and configure the network. e. Check components, connections, and cabling by generating test packets and tracking the results. f. Identify problem conditions by setting parameters to generate alerts. C. Network General Sniffer 1. Pair of a family of analyzers from Network General. 2. Decodes and interprets frames from 14 protocols including AppleTalk, Windows NT, NetWare, SNA, TCP/IP, VINES, and X.25. 3. Measures network traffic in kilobytes per second, frames per second, or as a percentage of available bandwidth. 4. Gathers LAN traffic statistics, detects faults such as beaconing, and presents this information in a profile of the LAN. 5. Identifies bottlenecks by capturing frames between computers and displaying the results. D. Novell’s LANalyzer 1. Performs like a Sniffer, but is available only on a NetWare LAN. 3. Monitoring and Troubleshooting Tools A. Network Management Overview 1. Administrator will need to manage and keep track of every aspect of the network’s performance. 2. Small peer-to-peer networks consisting of ten or fewer computers can be monitored visually by one support person. 3. A large network or WAN might need a dedicated staff and sophisticated equipment to perform proper network monitoring. 4. Observe certain aspects of the network’s day-to-day behavior to ensure the network does not fail. 5. Consistent monitoring of the network will reveal if any areas begin to show a decline in performance. 6. Scope of a network management program depends on: a. Size of the network b. Size and capabilities of the network support staff c. Organization’s network operating budget d. Organization’s expectation of the network B. Performance Monitors 1. Most NOSs include a monitoring utility that will help a network administrator keep track of a network’s server performance. 2. Monitors can view operations in both real time and recorded time for: a. Processors b. Hard disks c. Memory

Outline, Chapter 13 7 Networking Essentials Plus, Third Edition d. Network utilization e. The network as a whole 3. These monitors can: a. Record the performance data. b. Send an alert to the network manager. c. Start another program that can adjust the system back into acceptable ranges. C. Network Monitors 1. Some servers include network monitoring software. 2. Windows NT Server includes a diagnostic tool called Network Monitor, which gives the administrator the ability to capture and analyze network data streams. 3. The packets of data in a data stream consist of the following information: a. Source address of the sending computer b. Destination address of the receiving computer c. Headers from each protocol used to send the frame d. Data or a portion of the information being sent D. Simple Network Management Protocol (SNMP) 1. One of the network management standards created by network equipment vendors. 2. Programs called “agents” are loaded onto each managed device. 3. Agents monitor network traffic in order to gather statistical data. 4. Data is stored in a Management Information Base (MIB). 5. A management program console regularly polls these agents and downloads the information from their MIBs. 6. After the raw information has been collected, the management program performs two more tasks: a. Presents the information in the form of graphs, maps and charts. b. Sends the information to designated database programs to be analyzed. 7. If data falls above or below thresholds set by the manager, the management program can notify the administrator by means of alerts on the computer or by automatically dialing a pager number. 8. Support staff uses the management console program to implement changes. 9. SNMP components include: a. Hubs b. Servers c. NICs d. Routers and bridges e. Other specialized network equipment

8 Outline, Chapter 13 Networking Essentials Plus, Third Edition Chapter 13, Lesson 3 Where to Find Help 1. Network Support Resources A. It is as important to know when and where to get support in resolving network problems as it is to give support to others. B. It is impossible for any network administrator to know everything. C. Obtaining access to several resources is the key to successful network troubleshooting. D. Network support resources can be found in a wide range of locations: 1. Coworkers 2. Manufacturer and vendor help desks 3. CD technical references 4. Software products 5. Online support services 6. Print materials 7. Telephone support services 2. TechNet A. Microsoft Technical Information Network (TechNet) 1. Provides information to support all aspects of networking with an emphasis on Microsoft products. 2. With Microsoft KnowledgeBase one can find up-to-date articles on many topics and learn about the latest software releases, updates, and revisions. B. Installing TechNet 1. Installs just like any other program. 2. Adds icons to the appropriate program group for easy access. 3. TechNet Find tool is one of several methods of locating the information needed on TechNet. 4. Refer to the Help menu on the CD for more information on using the various features of TechNet. C. Subscribing to TechNet 1. On-line information about Microsoft TechNet: www.microsoft.com/technet/subscription/about.htm 2. The on-line version of TechNet: www.microsoft.com/technet/ 3. Subscribe to TechNet by calling: (800) 344-2121 3. Bulletin Board System (BBS) A. Many BBSs are devoted to technical subjects such as networking. B. Access the knowledge of experienced networking professionals by posting troubleshooting questions to them. C. BBSs are being replaced by the Internet and the Web.

Outline, Chapter 13 9 Networking Essentials Plus, Third Edition 4. User Groups A. Good source of information. B. Wealth of knowledge available at group meetings. C. Some meetings are held online; others meet in person. D. Tend to be organized around a particular operating system or hardware platform. E. Local addresses or Internet addresses for user groups can be found online or listed on the premises of local computer businesses. F. Allows a link to colleagues in your area. G. Can learn how others have solved similar problems and problems not yet encountered. 5. Periodicals A. Network technology is in a constant state of evolution. B. Many books are available on networking, but they quickly become outdated. C. Often a better resource than books because they provide the most current information available. D. Many periodicals and related resources are also available through the Internet. E. Can be helpful to maintain a catalog of articles from various periodicals. 6. The Internet: A Worldwide Resource A. Overview 1. Gives users access to information ranging from the latest technologies to product information to troubleshooting help. 2. Is a worldwide collection of networks, gateways, servers, and computers linked together using a common set of telecommunications protocols. 3. Provides worldwide access to vast stores of information resources that are easily accessible from universities, government organizations, the military, libraries, and other private and public organizations. 4. Evolved from a U.S. Department of Defense project, the Advanced Research Projects Agency Network (ARPANET), which was designed for packet-switching networks. 5. The protocol used for ARPANET was TCP/IP, which continues to be used on the Internet today. B. Services 1. Growing tremendously along with the services it provides. 2. Best-known services include: a. World Wide Web b. File Transfer Protocol (FTP) servers c. Electronic mail d. News e. Telnet C. World Wide Web

10 Outline, Chapter 13 Networking Essentials Plus, Third Edition 1. Contains a vast storehouse of hypertext documents written using the Hypertext Markup Language (HTML), Extensible Markup Language (XML), and Standard Generalized Markup Language (SGML), among others. 2. Hypertext is a method for presenting text, images, sound, and videos that are linked together in a nonsequential web of associations. 3. Hypertext format allows the user to browse through topics in any order. 4. Internet tools and protocols help to locate resources and transport them between computers. 5. Most network and electronics manufacturers maintain Web sites. 6. Some of the better sites will contain a complete, searchable knowledge base. 7. Becoming familiar with the Web sites of network manufacturers can greatly expand resources for network maintenance, upgrading, and troubleshooting. 8. A good corporate Web site will have: a. FAQs (frequently asked questions and answers). b. Updated technical information about their products. c. Updated software drivers (if their products use them). d. E-mail addresses to which users can write to get answers to questions. D. Domain Name System (DNS) 1. IP address and domain names. a. Every computer on the Internet has a unique IP address. b. IP address is four sets of digits separated by dots (for example, 198.46.8.34). c. DNS was created because these strings of numbers were hard to remember and difficult to accurately type. d. Domain names enable short, alphabetical names to be assigned to IP addresses to describe where a computer is located. e. In the example http://www.microsoft.com, the domain name is www.microsoft.com. f. Domain names can represent the corporate identity, as is the case with Amazon.com, or become part of it, as is the case with cnn.com. g. A Web site with a recognizable or memorable name will receive more hits than a site with an unimaginative name. 2. Microsoft Windows NT Server HOSTS and LMHOSTS files. a. Configuration files that are responsible for name resolution. b. ASCII files that can be edited with Microsoft Notepad or any other plain-text editor. c. HOSTS file resolves host names to IP addresses. d. LMHOSTS file resolves NetBIOS names to IP addresses. 3. The last three characters of the DNS or UNC name indicate the type of domain. a. Domain types common in the United States include the following:

Outline, Chapter 13 11 Networking Essentials Plus, Third Edition (1) Commercial organizations: .com (2) Educational institutions: .edu (3) Governmental organizations (except the military): .gov (4) Military organizations: .mil (5) Network service providers: .net (6) Organizations (such as nonprofit groups): .org b. International domain designations include the following: (1) Australia: .au (2) France: .fr (3) United Kingdom: .uk (4) United States: .us E. Internet Names 1. Resources on the Internet have their own location identifier or Uniform Resource Locator (URL). 2. URLs specify the server to access, the access method, and the location. 3. Address begins with two forward slashes. 4. URL is very similar to the Universal Naming Convention (UNC) except the URL begins with two forward slashes (//) and UNC begins with two backslashes (\\). 5. URL consists of several parts. 6. Simplest version contains: a. The protocols to be used (http) b. A colon (:) c. The address of the resource (//www.microsoft.com) F. File Transfer Protocol (FTP) 1. Most common protocol used for sending files between computers. 2. One method for supporting remote networks. 3. FTP servers can provide vast amounts of information stored as files. 4. Data in these files cannot be accessed directly; the entire file must be transferred from the FTP server to the local computer. 5. Designed for TCP/IP environments and is implemented at the application layer of the OSI model. 6. Microsoft Windows NT, Windows 95 and Windows 98, and Windows 2000 include the traditional character-based FTP client. 7. FTP utility is copied onto the system when the TCP/IP protocol suite is installed. 8. Most Internet browsers, such as Opera, Netscape, and Microsoft Internet Explorer, support FTP and use it to transfer files. 9. Manufacturers will often refer download file requests to their FTP site. 10. FTP is much faster and better for file transfers and it keeps Web sites open for other users. G. Electronic Mail (E-mail) 1. The sending and receiving of electronic messages; currently one of the most popular activities on the Internet.

12 Outline, Chapter 13 Networking Essentials Plus, Third Edition 2. For many people e-mail is the primary reason for getting onto the Internet or subscribing to an online service. 3. Must know the recipient’s e-mail address. 4. Addresses are composed of the user’s identification, followed by the @ sign, followed by the location of the recipient’s mail server. 5. Can be exchanged without incurring the expense of a long-distance telephone call. 6. Has the advantage of allowing access to e-mail messages at the user’s convenience. 7. Can send an identical message to multiple recipients at one time. H. News 1. Network News Transfer Protocol (NNTP) is an Internet standard protocol defined for distribution, inquiry, retrieval, and posting of news articles. 2. Network News (USENET) is a popular use of NNTP. 3. Network News is a massive system with over 5,000 ongoing conferences, called newsgroups, conducted 24 hours a day, 365 days a year. 4. To access newsgroups, download a special program from the Internet that allows participation in any newsgroup. 5. Most commercial browsers, including Microsoft Internet Explorer, have this capability built-in. 6. Subscribe to newsgroups of interest and communicate through a message system similar to e-mail. 7. Network News differs from e-mail because conversations take place in a public forum called a newsgroup. 8. Viewing an ongoing dialog without participating is called lurking and is encouraged for newcomers. 9. Post an article in the newsgroup to become part of the forum. 10. Network News operates at a very high speed, with postings appearing quickly and continuously. 11. Group administrators set the length of time that messages can remain posted before being deleted from the system. 12. Most do not keep postings longer than a week. 13. Discussion groups and chat rooms can be excellent sources of information and assistance on technical issues. I. Telnet 1. One of the first Internet protocols developed and can be used to act as a remote terminal to an Internet host. 2. When connected to an Internet host, the computer acts as if the keyboard were attached to the remote computer. 3. Can run programs on a computer on the other side of the world, just as if the operator were sitting in front of it. 4. Terminal/host system evolved from UNIX character-based systems in the early days of the Internet.

Outline, Chapter 13 13 Networking Essentials Plus, Third Edition 5. Microsoft Windows 95 and Windows 98, Windows NT, and Windows 2000 install a Telnet program as part of the TCP/IP utilities. 6. Acts as either a VT-52 or VT-100 terminal to a system accessible by means of TCP/IP, including computer reached by means of the Internet. J. Internet Sites 1. Many companies offer various types of support through Internet sites. 2. Administrators can turn to these sites for help with network problems. 3. Include services that often provide information that will help solve particular network problems. 4. Subscription support services are available through companies such as Microsoft. K. Locating Resources 1. If the URL for the vendor is unknown, try typing in vendorname.com. 2. Use a search engine to look for the company name. 3. Look through all of the results; the information might not be on the first page of the results. 4. If search yields hundreds or thousands of results, try refining the search. 5. Locating resources on the Internet is possible because each resource on the Internet has an address. 6. Browser programs use these resource addresses and search engines to help users find information on a specific topic. L. Browsers 1. A graphical interface called Web browser is needed to view the Web. 2. Some common Web browsers are Mosaic, Netscape, and Microsoft Internet Explorer. 3. Search sites have proliferated to help users access the wealth of information that continues to appear on the Internet. 4. Some of the more common search sites are: a. AltaVista http://www.altavista.com b. Deja http://www.deja.com c. Google http://www.google.com d. GoTo http://www.goto.com e. HotBot http://www.hotbot.com f. InfoSeek http://infoseek.go.com g. Lycos http://www.lycos.com h. Netscape Search http://www.home.netscape.com i. Yahoo! http://www.yahoo.com M. Making an Internet Connection 1. Dial-Up a. Several variations on the dial-up account provide different capabilities, depending on the protocols used. b. All these connections require the Internet Protocol and are called IP accounts.

14 Outline, Chapter 13 Networking Essentials Plus, Third Edition c. Three types of accounts are: Point-to-Point Protocol (PPP), Serial Line Internet Protocol (SLIP), and CSLIP, which is a compressed version of SLIP. d. PPP is the emerging connection of choice because it is faster and more reliable that other IP account types. e. PPP is more complex and many computer platforms still only have built-in support for SLIP. f. Supplemental programs are being developed to enable most platforms to support PPP accounts. g. PPP offers a dynamic allocation of IP addresses similar to Dynamic Host Configuration Protocol (DHCP), which makes logging on to the service simpler than having to provide a valid IP address. h. With both SLIP and PPP, the provider gives the user a temporary IP address on the Internet. i. Commercial online services, such as the Microsoft Network and America Online, often make a Winsock/PPP access method available either as part of their service or as an additional service for a supplemental fee. 2. ISDN a. Telecommunications service that connects networks through digital lines using a terminal adapter. b. Provides a faster connection and can be more economical than dial- up service. 3. Considerations. a. Both the dial-up and the ISDN methods can connect single-user accounts or multiple-user accounts to the Internet. b. Dial-up accounts are more affordable for individual users. c. ISDN provides a more economical solution for LANs, which connect multiple users at a specific location to the Internet. d. If connected directly to the Internet or to a service provider, the computers on the Internet are essentially a part of the WAN, which means they can be accessed directly. e. One issue is that information on each computer is also accessible, which can lead to potential security concerns. f. It is common for organizations using a direct connection to set up a special machine, called a proxy agent, to act as a gateway between their local network and the Internet. g. The proxy agent filters requests over the gateway and makes it more difficult for unauthorized requests to reach the local network.

Outline, Chapter 13 15 Networking Essentials Plus, Third Edition