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A+ Study Guide: Domain 5.0: Networks - Troubleshooting Networks

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Basic Troubleshooting Steps
Troubleshooting Tools
The OSI Model

In this section, we will take a look at some common networking problems and how to perform basic troubleshooting on them. As you read this guide, keep the following in mind:
  • An A+ technician is not a network administrator, so the troubleshooting a person in this position would do should be very basic.
  • This is by no means a complete list of problems and solutions you may encounter.
  • A good troubleshooter needs common sense, logic, and the ability to narrow problems down. This guide cannot provide you with that.
Investigating the Problem:
Before you can fix a problem, you need to gather information to understand and verify it. Here are a few common steps to begin the troubleshooting process.
  • Verify the Problem - Ideally if you are at the customer's location, you can have them show you what is happening so that you can see the steps they are taking to create the undesired behavior. However, in many cases, you might be on the phone in another location and unable to do this. In these situations, you need to walk the customer through the proper steps for the task they are trying to accompish and see where the problem is occuring. Clear communication skills are very important and you should do your best to describe the proper steps for the customer to take. In many case, you may find there was no problem other than the user not knowing how to do what they wanted to do.
  • When did it start - If you verify that there is a problem, it is a good idea to determine when the problem began occuring. This can help if you are aware or become aware of changes that have been made. If the problem only happened once, there may not be a problem.
  • What has changed - Related to the above, you should try to find out if anything has changed around the time frame that the problem began occuring. Once the user has identified when the problem began, you can ask them to think of anything they or their company may have done recently that might be responsible.
  • Reproducing the problem - Even if you have had the customer reproduce the problem over the phone with you, it is a very good idea to see it with your own eyes if possible.
  • Narrowing down the problem - At this point, you have verified that there is indeed a problem and have seen the behavior. Next begins the process of narrowing down the problem. How this happens will depend on the specific problem and you (see the 3rd bullet in the Introduction above). Things like narrowing the problem down between hardware and software, for example, can lead you down the path to the solution.
Troubleshooting Tools:
Of course, if you are going to solve a problem, you will need some tools to help you. Some of these tools are hardware and some software. Let's take a look at them..

On the hardware end, it is always good to keep spare parts such as network cards, patch cables, a hub, etc. for troubleshooting purposes. These components do occasionally go bad or get damaged. Just like working on PC hardware, swapping out components can help narrow down, or even solve the problem. A more expensive troubleshooting tool is a network sniffer (such as a Fluke tool) that can isolate network problems.

And now for some of the command line network tools.
  • IPCONFIG - This utility allows you to quickly check the TCP/IP configuration of a computer from the command line. When used with the /all switch (ipconfig /all), virtually every TCP/IP setting is displayed including the IP address, subnet mask, default gateway, DNS servers, MAC address, and more. When having a network problem, this is one of the most useful tools for checking for configuration problems.
  • PING (Packet InterNet Groper) - PING is a command-line utility used to verify connections between networked devices. PING uses ICMP echo requests that behave similarly to SONAR pings. The standard format for the command is ping ip_address/hostname. If successful, the ping command will return replies from the remote host with the time it took to receive the reply. If unsuccessful, you will likely recieve and error message.
  • NSLOOKUP - This is a command that queries a DNS server for machine name and address information. Originally written for Unix operating systems, this command is now available on Windows and other operating systems. To use nslookup, type "nslookup" followed by an IP address, a computer name, or a domain name. NSLOOKUP will return the name, all known IP addresses and all known aliases (which are just alternate names) for the identified machine. NSLOOKUP is a useful tool for troubleshooting DNS problems.
  • TELNET - Telnet is a protocol that allows you to connect to remote computers over the Internet. There is a telnet server on the remote host, which the telnet client negotiates with to establish a connection. Once connected, the client becomes a virtual terminal, and allows you to interface with the host computer via command line from your computer. In most cases, you'll be asked to log into the remote system. This usually requires an account on that host. Telnet uses port 23. Because of security issues with Telnet, Secure Shell (SSH) is replacing becoming the new standard.
  • TRACEROUTE - A command-line troubleshooting tool that enables you to view the route to a specified host. This will show how many hops the packets have to travel and how long it takes. In Windows operating systems, the command used is "TRACERT".
The OSI Model:
OSI Model (Open Systems Interconnection Model) was developed by the International Standards Organization (ISO). The OSI reference model is a 7 layer networking framework that divides the networking process into logical layers which are often used as a framework for troubleshooting. The 7 layers are: Physical, Data Link (which has sub-layers called Media Access Control and Logical Link Control), Network, Transport, Session, Presentation, and Application. You won't need to know in-depth information about this for the exam, but you should be familiar with it.