Resolving IP Addressing Problems

The current IP address settings of a computer can be obtained as discussed in the "Viewing Network Configuration Information" section of this tutorial. If a computer is having problems accessing network resources or communicating with other computers, an IP addressing problem might exist. Take a close look at the IP address currently assigned, as well as other IP address settings, and use these pointers to help in your troubleshooting:

• If the IPv4 address currently assigned to the computer is in the range 169.254.0.1 to 169.254.255.254, the computer is using Automatic Private IP Addressing (APIPA). An automatic private IP address is assigned to a computer when it is configured to use DHCP and its DHCP client cannot reach a DHCP server. When using APIPA, Windows Vista will automatically periodically check for a DHCP server to become available. If a computer doesn't eventually obtain a dynamic IP address, it usually means the network connection has a problem. Check the network cable, and if necessary trace the cable back to the switch or hub into which it connects.
• If the IPv4 address and the subnet mask of the computer are currently set as 0.0.0.0, the network is either disconnected or someone attempted to use a static IP address that duplicated another IP address already in use on the network. In this case, you should access Network Connections and determine the state of the connection. If the connection is disabled or disconnected, this should be shown. Right-click the connection and select Enable or Repair as appropriate. If the connection is already enabled, you will need to modify the IP address settings for the connection.
• If the IP address is dynamically assigned, check to make sure that another computer on the network isn't using the same IP address. You can do this by disconnecting the network cable for the computer that you are working with and pinging the IP address in question. If you receive a response from the PING test, you know that another computer is using the IP address. This computer probably has an improper static IP address or a reservation that isn't set up properly.
• If the IP address appears to be set correctly, check the network mask, gateway DNS, and WINS settings by comparing the network settings of the computer you are troubleshooting with those of a computer that is known to have a good network configuration. One of the biggest problem areas is the network mask. When subnetting is used, the network mask used in one area of the network might look very similar to that of another area of the network. For example, the network mask in one IPv4 area might be 255.255.255.240, and it might be 255.255.255.248 in another IPv4 area.