Posted by arlene

If a client is configured to use DHCP, what happens if no DHCP server is available on the network? Starting with Windows 98, Microsoft Windows clients can use Automatic Private IP Addressing (APIPA). This is not a solution for a large network. It is for use on small LANs, such as a home office with 25 or fewer network nodes.

Simply configure each client computer to use DHCP in the properties page for TCP/IP, and reboot. When the client computer realizes that no DHCP server is on the network (because it’s not receiving any replies from its broadcasts), it will timeout and begin to use APIPA. The scheme in which addresses are allocated is not that complicated.

The network of addresses reserved for use by APIPA is 169.254.0.1 to 169.254.255.254, with a subnet mask of 255.255.0.0. When the client does not receive an answer from any DHCP server after a short time, it will select an address randomly from this network. It then will test to see whether this address is already in use.

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Note that a Microsoft client that is using APIPA will periodically check the network (about every five minutes) to see whether a DHCP server has become available. If one does come online, the client will perform as any other DHCP client and obtain configuration information from the DHCP server it discovers. Some older versions of Windows, notably Windows 98, do not automatically check for DHCP servers. In such cases, after you solve the problem that caused the computer not to find the DHCP server (such as a loose or damaged network cable), restart the computer. The computer should receive a valid IP address from the DHCP server after rebooting.

Because clients randomly choose IP addresses, it is always quite possible that one computer will choose an address already in use. To solve this problem, each computer first chooses an address and then broadcasts a packet containing that IP address and waits to see whether another computer replies that the address is already in use. This is referred to as gratuitous ARP. When this occurs, a client will attempt to select an IP address up to 10 times before ending the process.

Tip

If a Windows 2000 or Windows XP client is unable to receive a valid IP address from a DHCP server, but other devices connected to the same DHCP server are working correctly, check physical connections to the network and restart the connection and then the computer. If you are still unable to make a connection, make sure the malfunctioning client’s DHCP client service is running.

To check the status of a computer’s DHCP client service, follow these steps:

1. Right-click My Computer and select Manage.
2. Click Services and Applications, Services.
3. Scroll down to DHCP Client. Make sure the status is listed as Started and the startup type is Automatic.
4. To start the service or change the startup type, double-click DHCP Client. Click Start to start the client and select Automatic as the startup type.
5. Click Apply, and then OK.

Creating a Manual IP Address As an Alternative to APIPA

APIPA can be useful in creating a small LAN that does not need access to the Internet or another network. It can also be confusing, however, if your computers can connect to each other but not the Internet. As an alternative to using APIPA, you can assign an alternate manual IP address to computers running Windows 2000 or Windows XP. The alternate address is used if the computer is unable to connect to a DHCP server. Follow this procedure:

• Open the properties sheet for the current network connection.
• Click the General tab.
• Select Internet Protocol (TCP/IP) and click Properties.
• Click the Alternate Configuration tab. Select User Configured.
• Enter the IP address, subnet mask, default gateway, and DNS and WINS server information for the network client. Note that the IP address must be unique for each client, but the other information is the same for each user on the network.
• Click OK when finished.

If the client is unable to connect to a DHCP server, it uses the alternate manual address instead. This can keep your network running if the DHCP server fails but a different device or server is responsible for routing traffic to other networks or the Internet.

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