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Automatic Windows 98/ME TCP/IP Addressing Without a DHCP Server

Posted On 2005-11-1 by FortyPoundHead
Keywords: Automatic Windows 98/ME TCP/IP Addressing Without a DHCP Server
Tags: Networking Tip Software Hack/Tweak Windows
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A Windows based computer that is configured to use DHCP can automatically assign itself an internet protocol (IP) address if a DHCP server is not available. For example, this could occur on a network without a DHCP server, or on a network if a DHCP server is temporarily unavailable. The IP address assigned through APIPA comes from a pool of private, internal IP addresses that are not valid for computer that are participating on the Internet. The addresses are in the class b IP address range, and hosts are assigned an IP address from the following network address range:
169.254.3-digit-number.3-digit-number
where 3-digit-number is a 3 digit number. After the network adapter has been assigned an IP address, the computer can use the TCP/IP to communicate with any other computer that is connected to the same LAN and that is also configured for APIPA or has the IP address manually set to the 169.254.3-digit-number.3-digit-number address range with a subnet mask of 255.255.0.0. Note that the computer cannot communicate with computers on other subnets, or with computer that do not use automatic private IP addressing. Automatic private IP addressing is enabled by default. You may want to disable it in any of the following cases:

  • Your network uses routers.

  • Your network is connected to the Internet without a NAT or proxy server.

Unless you have disabled DHCP-related messages, DHCP messages provide you with notification when you change between DHCP addressing and automatic private IP addressing. If DHCP messaging is accidentally disabled, you can turn the DHCP messages back on by changing the value of the Popupflag value in the following registry key from 00 to 01:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\DHCP

Note that you must restart your computer for the changes to take effect. You can also determine whether your computer is using automatic private IP addressing by using the Winipcfg tool:

Click Start, Run, type winipcfg, and click OK.
Click More Info. If the IP Autoconfiguration address box contains an IP address within the 169.254.x.x range, automatic private IP addressing is enabled. If the IP address box exists, automatic private IP addressing is not currently enabled.

You can disable the automatic private IP addressing by using either of the following methods:

You can configure the TCP/IP information manually which disables DHCP altogether.
You can disable automatic private IP addressing (but not DHCP) by editing the registry. You can do so by adding the “IPAutoConfigurationEnabled” DWORD registry entry with a value of 0x0 to the following registry key:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\DHCP

Examples of Where APIPA May Be Useful

Example 1: No Previous IP Address and no DHCP Server

When your Windows ME/98 based computer (configured for DHCP) is initializing, it broadcasts three or more “discover” messages. If a DHCP server does not respond after several discover messages are broadcast, the Windows ME/98 computer assigns itself a class B (APIPA) address. Then the computer will display an error message to the user of the computer (providing it has never been assigned an IP address from a DHCP server in the past). The computer will then send out a discover message every three minutes in an attempt to establish communications with a DHCP server.

Example 2: Previous IP Address and no DHCP Server

The computer checks for the DHCP server and if none are found, an attempt is made to contact the default gateway. If the default gateway replies, then the Windows computer retains the previously-lease IP address. However, if the computer does not receive a response from the default gateway, or if none are assigned, then it uses the automatic private IP addressing feature to assign itself an IP address. An error message is presented to the user and discover messages are transmitted every three minutes. Once a DHCP server comes on line, a message is generated stating communications have been re-established with a DHCP server.

Example 3: Lease Expires and no DHCP server

The Windows ME/98 based computer tries to re-establish the lease of the IP address. If the Windows computer does not find a DHCP server, it assignes itself an IP address after generating an error message. The computer then broadcasts a discover message every three minutes until a DHCP server comes on line. A message is then generated stating that communications has been restored with the DHCP server.


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