Addressing TCPIP Problems

Addressing TCP/IP Problems

Because TCP/IP is the communications protocol on which the very lifeblood of a home network depends, it stands to reason that if there is a problem at the TCP/IP level, you will not be able to get to anything outside your local computer. The symptoms you can expect to see when you experience problems with TCP/IP include an inability to use your web browser to connect to websites, an inability to connect to your shared video or audio library, and an inability to use shared files and printers.

The first thing to check in these cases is the physical connection between your computers. If your computers are networked using wires, you need to ensure that they are connected properly. If you are using wireless networking, you need to ensure that your wireless card is pushed in all the way and that your wireless access point is powered up. If the physical elements of your network are all in place and working correctly, you can use some of the invaluable tools that Windows provides for diagnosing and solving this problem. The following sections describe the Network Diagnostics tool and some of the available Windows command-line tools.

The Network Diagnostics Tool

The easiest method of troubleshooting network connectivity issues is by starting the Network Diagnostics tool, which is available in the Windows Help and Support Center. You can run this graphical tool to analyze various components of network connectivity and get a report that shows the results. To run the Network Diagnostics tool, follow these steps:

1.	Select Start, Help and Support. The Help and Support Center appears.
2.	Under Pick a Task, click Use Tools to View Your Computer Information and Diagnose Problems. Note You can also run the Network Diagnostics tool by selecting Start, Run and then typing netsh diag gui and clicking OK.
3.	Under Tools, click Network Diagnostics, and then in the right side of the screen, click Scan Your System. A report like the one shown in Figure 7.1 appears. Figure 7.1. The Network Diagnostics tool.
4.	Under Modems and Network Adapters in the Network Diagnostics tool report, click Network Adapters. The details of your network adapter appear. Look for the word FAILED (in red text) next to any item in this list. You can click to expand any item marked this way to get more information.

In the example shown in Figure 7.2, there is a problem with the default gateway. This example is from my home network and is a good example of needing some understanding of the network in order to interpret results. On my network, the 10.0.0.1 address is a firewall computer that is acting as my gateway to the Internet as well. I have intentionally turned off the ability to process ICMP packets on my firewall as a security measure. So in the case of this network, this is not really a problem, as long as we understand what is happening.

Figure 7.2. Network Diagnostics tool results.

The Network Diagnostics Tool

The Network Diagnostics tool gathers information about your computers to help you solve network problems. It also lets you run a variety of tests to gather information to help you solve problems with your network. The program scans your computer to see if you have network connectivity and whether the programs required for proper network functions are running on your computer. The output of the tool is a graphic representation of information to help you quickly identify what might be wrong with the networking components of your network. All scans are performed only on your local computer to maintain privacy and the security of your data.

Command-Line Tools

Those of us who have been around networking long enough remember running applications and utilities from a somewhat archaic command interpreter. In DOS it was the famous C:> prompt, and in UNIX it was the shell prompt, which would vary, depending on the shell of your choice (for example, the C shell, Bourne shell, or Korn shell). Under both UNIX and DOS, interaction with the computer was basically the same: You typed a command and pressed Enter, and the computer displayed the results in text and/or numeric form. There were no windows or pretty pictures and icons; you simply got a response to the command that you gave to the interpreter. In those days, the ping utility was extremely useful as a network troubleshooting tool. ping is still available and is still just as useful as ever.

You use the ping utility to send Internet Control Message Protocol (ICMP) packets to the intended destination. If the destination replies, you know that the connection between the two computers is working. You use the ping utility at a command prompt, which you can access on a Windows system by selecting Start, All Programs, Accessories. Figure 7.3 shows an example of using the ping utility.

Figure 7.3. The ping utility.

If you see replies to your ping, you know that the two computers can communicate just fine, and you can move on and focus your troubleshooting efforts elsewhere.

However, if all the replies show up as "Request timed out," then either your computer or the destination computer has a TCP/IP problem. To narrow down where the problem is, you should check network connectivity, moving from your local computer outward, until you either encounter success or find a point where you are not able to communicate any further. This is sort of analogous to the "get your own house in order first" school of thought. In general, you should follow these steps and stop wherever you run into a problem: While this approach may not work for more complex environments, for a home network, it should get you up and running. Firewalls and packet-filtering routers and switches complicate this simplistic approach to troubleshooting.

1. ping your own computer first, using one of the following commands:

   ping 127.0.0.1
   ping localhost

   
   

   Note

   Each computer on a TCP/IP network maintains what is called the local loopback address. This address is expressed as 127.0.0.1 and is also known by the name localhost and can be used as a way to test whether TCP/IP is loaded and working properly on your local computer. If a ping to localhost or 127.0.0.1 fails, there is a problem with the TCP/IP drivers loaded on your machine, and you probably need to reinstall them. This is not as catastrophic as it might seem. You can do it by selecting Start, Run and then typing netsh int ip reset resetlog.txt and clicking OK.

   
   

2. ping your computer's IP address.

   Tip

   You can find the IP address for the computers in your network by typing ipconfig /all in a command prompt window. Figure 7.4 shows an example of the output from ipconfig. Among the items you'll see are the computer's IP address, the default gateway IP address, and DNS and WINS server information. If your computer shows an IP address in the form 169.254.x.y, your computer is using Automatic Private IP Addressing (APIPA). This means that your computer attempted to use Dynamic Host Configuration Protocol (DHCP) to obtain an IP address but was unable to do so because it was unable to contact a DHCP server. See the section "Fixing IP Address Problems," later in this chapter, for more information.

   Figure 7.4. ipconfig output

   
   
   

3. ping the IP address of another computer on your network. If you receive no response from the other computer but have been successful to this point, the problem is likely with the remote computer.

   Note

   Try to ping multiple computers before you conclude that the problem is with remote computers.

   
   

4. ping the address of your default gateway. This may or may not be useful. In general, if your gateway responds to ping requests, you can check along the communications path to ensure that data is reaching its destination. Nowadays, however, these types of devices often do not respond to ping requests as a security measure.

5. ping a well-known site on the Internet, such as www.google.com or www.microsoft.com. Try several sites before concluding that there is a problem in this step.

   Note

   Recall from earlier in this chapter that my default gateway would not respond to ping packets when I used the Network Diagnostics tool. Many sites on the Internet employ the same policy, as a security measure, and you may not get any response whatsoever when attempting to ping them. This is why you should try multiple destinations before coming to a conclusion.

   
   

   If you don't find a problem until step 5, your problem is not a TCP/IP problem, and you should next look for domain name system (DNS) problems, as described later in this chapter, in the section "Diagnosing and Fixing DNS Problems."