Diagnosing IP Address Problems In Microsoft Windows XP

On most networks, IP addresses are assigned automatically by Dynamic Host Configuration Protocol (DHCP) servers; in some cases, you need to use static IP addresses, which are fixed numeric addresses. Problems with DHCP servers or clients can cause network connections to stop working, as can incorrectly assigned static IP addresses.

To see details of your current IP configuration, follow these steps:

1. Open Control Panel and double-click Network Connections (click Network And Internet Connections if you’re using Control Panel’s Category view).

2. Double-click the icon for the connection about which you want more information. (Alternatively, you can select the icon and click the View Status Of This Connection link in the Network Tasks pane.)

3. Click the Support tab to see the currently assigned IP address, subnet mask, and default gateway for that connection, as in the example in Figure 1.

Figure 1. This IP address was automatically assigned by the DHCP server in a router.

If those details aren’t sufficient—in particular, if you want to see information about DNS servers and when your DHCP address will expire and be renewed—click the Details button. Figure 2 shows the results.

Figure 2. This dialog box provides extra information about your network connection. Normally, DHCP addresses are renewed automatically at intervals shown here.

Want still more information? Then fire up a Command Prompt window (Cmd.exe) and use the IP Configuration utility, Ipconfig.exe. Used without any parameters, typing ipconfig at a command prompt displays the DNS suffix, IP address, subnet mask, and default gateway of your primary network connection (identified as Local Area Connection in the Network Connections folder, unless you’ve renamed it). To see exhaustive details about every available network connection, enter ipconfig /all.

The actual IP address you see may help you solve connection problems:

● If the address is in the format 169.254.x.y, your computer is using Automatic Private IP Addressing (APIPA). This means your computer’s DHCP client was unable to reach a DHCP server to be assigned an IP address. Check the connection to your network.

● If the address is in one of the blocks of IP addresses reserved for use on private networks (for details, see “Setting IP Addresses,” page 971), make sure that another computer (an Internet Connection Sharing host) or a router or residential gateway is routing your Internet requests to a properly configured public IP address.

● If the address of your computer appears as 0.0.0.0, the network is either disconnected or the static IP address for the connection duplicates an address that already exists on the network.

● Make sure you’re using the correct subnet mask for computers on your local network. Compare IP settings on the machine that’s having problems with those on other computers on the network. The default gateway and subnet mask should be identical for all network computers. The first one, two, or three sets of numbers in the IP address for each machine should also be identical, depending on the subnet mask. A subnet mask of 255.255.255.0 means the first three IP address numbers of computers on your network must be identical—192.168.0.83 and 192.168.0.223, for instance, can communicate on a network using this subnet mask, but 192.168.1.101 will not be recognized as belonging to the network. Likewise, with a subnet mask of 255.255.0.0, the first two numbers in each address must match—172.16.2.34, 172.16.4.56, and 172.16.83.201 are all valid addresses on a subnet with this mask. In every case, the gateway machine must also be a member of the same subnet. (If you use a router, switch, or residential gateway for Internet access, the local address on that device must be part of the same subnet as the machines on your network.)