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An IP address uniquely identifies computers and computing devices, such as the cable modem, on a TCP/IP network. IP addresses are normally expressed in dotted-decimal format, with four numbers (or “octets”) separated by periods, such as 192.168.100.1. For troubleshooting purposes, all the numbers in the IP addresses of your networked devices must be consistent across the network. This means:

Troubleshooting Procedures

Ÿ The last three numbers (i.e., the last octet) in the IP addresses are different and not in use by other devices on the network (assuming your subnet mask is

255.255.255.000).

For example, if the IP address for the cable modem is 192.168.100.1, the other devices on the network must have an IP address whose:

Ÿ First three octets are 192.168.100.

Ÿ Last octet is any unique number from 0 to 255 (there are restrictions on using 0 and 255, so avoid using them). You would not use 1 as the last number, since that number is the last octet in the cable modem IP address in our example.

Figure 5-2 shows an example of IP addresses assigned to devices on the network.

Figure 5-2. Example of IP Addresses for Networked Devices

You define the computer’s IP address using the procedure appropriate for your operating system, as described in Chapter 3.

Tip: The following Windows sections describe how to use the command-line program

Windows IP Configuration (ipconfig) to see the IP address of a computer running a Windows operating system. However, if you want even more information, such as IP routing and DHCP information, type ipconfig /all instead of ipconfig in the Windows sections that follow.

Troubleshooting Procedures

The most common problem associated when viewing IP addresses is that either:

Ÿ A computer’s IP address is not on the same subnet as the other networked devices. In other words, the first nine numbers (three octets) in the device’s IP address are not identical to those of the other networked devices.

Ÿ A computer shows an IP address that starts with the digits 169.254. When this happens, it often means the computer could not retrieve a network address from a DHCP server and therefore automatically assigned itself an address from the base network address 169.254. This would be fine if all the computers on your network used IP addresses from the 169.254 base network address. If one of them isn’t, however, that computer will have problems connecting to the other network computers.

Possible causes for an IP address not being assigned are:

Ÿ Defective cables. Replace the cable if it is worn or defective.

Ÿ An Ethernet cable may not be firmly connected at both ends. Secure the cable at both ends. Often, just reseating the cable into a connector is all that is required.

Ÿ The NIC may be bad or the network cable connected to it may be the wrong kind (for example, it might be a straight-through cable when a crossover cable is required). If you have another NIC and cable available, try using them instead.

Ÿ The DHCP server that would automatically assign IP addresses is not configured for DHCP.

The sections that follow describe how to identify the IP addresses on devices running Windows and Macintosh operating systems.

Microsoft Windows 2000

To find the IP address of a computer running Microsoft Windows 2000: 1. From the Windows task bar, click Start and select Run.

2. In the Open field, type cmd and click OK. A DOS command window appears.

3. In the DOS command window, type ipconfig and then press Enter. Your IP address will be listed (see Figure 5-3).

Troubleshooting Procedures

Figure 5-3. Example of an IP Address Microsoft Windows XP, Vista, and 7

To find the IP address of a computer running Microsoft Windows XP, Vista, or 7: 1. Click on Start and in the Start Search box type cmd.

2. Press Enter on your keyboard or click cmd.exe in your search list (see Figure 5-4).

Figure 5-4. Example of md.exe in the Search List

3. In the DOS command window, type ipconfig and then press Enter. Your IP address will be listed (see Figure 5-3).

4. When you finish, type exit at the command prompt and press Enter to close the window.

Troubleshooting Procedures

Apple Macintosh OS X

To find the IP address of a Mac OS X OS X 10.6 or 10.5 computer: 1. From the Apple menu, select System Preferences...

2. In System Preferences, from the View menu, select Network.

3. In the Network preference window, click a network port (for example, Ethernet, AirPort, modem). If you are connected, the IP address appears below Status.

4. Close the window when finished.

To find the IP address of a Mac OS X 10.4 or 10.3 computer:

1. From the Apple menu, select Location, and then Network Preferences...

2. In the Network Preference window, next to Show, select Network Status. Your network status and IP address appear.

3. Close the window when finished. Via the Web

There are various Web sites, such as http://whatismyipaddress.com/, that display your IP address and other information about your system. If the page does not display, you may not have a working Internet connection and should try one of the methods described above to find your IP address.