Microsoft KB Archive/928233

Knowledge Base

Windows Vista cannot obtain an IP address from certain routers or from certain non-Microsoft DHCP servers

Article ID: 928233

Article Last Modified on 9/6/2007

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APPLIES TO

• Windows Vista Business
• Windows Vista Enterprise
• Windows Vista Home Basic
• Windows Vista Home Premium
• Windows Vista Ultimate
• Windows Vista Enterprise 64-bit Edition
• Windows Vista Home Basic 64-bit Edition
• Windows Vista Home Premium 64-bit Edition
• Windows Vista Ultimate 64-bit Edition
• Windows Vista Starter

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Important This article contains information about how to modify the registry. Make sure to back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows registry

SYMPTOMS

Consider the following scenario:

• You connect a Windows Vista-based computer to a network.
• A router or other device that is configured as a Dynamic Host Configuration Protocol (DHCP) server is configured on the network.
• The router or the other device does not support the DHCP BROADCAST flag.

In this scenario, Windows Vista cannot obtain an IP address.

CAUSE

This issue occurs because of a difference in design between Windows Vista and Microsoft Windows XP Service Pack 2 (SP2). Specifically, in Windows XP SP2, the BROADCAST flag in DHCP discovery packets is set to 0 (disabled). In Windows Vista, the BROADCAST flag in DHCP discovery packets is not disabled. Therefore, some routers and some non-Microsoft DHCP servers cannot process the DHCP discovery packets.

RESOLUTION

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.To resolve this issue, disable the DHCP BROADCAST flag in Windows Vista. To do this, follow these steps:

1. Click Start, type regedit in the Start Search box, and then click regedit in the Programs list.
   
   If you are prompted for an administrator password or for confirmation, type your password, or click Continue.
2. Locate and then click the following registry subkey:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{GUID}

   In this registry path, click the (GUID) subkey that corresponds to the network adapter that is connected to the network.
3. On the Edit menu, point to New, and then click DWORD (32-bit) Value.
4. In the New Value #1 box, type DhcpConnEnableBcastFlagToggle, and then press ENTER.
5. Right-click DhcpConnEnableBcastFlagToggle, and then click Modify.
6. In the Value data box, type 1, and then click OK.
7. Close Registry Editor.

MORE INFORMATION

If the router or the non-Microsoft DHCP server does not support the DHCP BROADCAST flag, you can set the following registry entry as follows:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{GUID}

Value name: DhcpConnForceBroadcastFlag
Value type: REG_DWORD
Value data: 0

Note A data value of 0 disables this registry entry. You can use this registry entry to prevent Windows Vista from using the DHCP BROADCAST flag. After you set this registry entry, Windows Vista never uses the DHCP BROADCAST flag.

Keywords: kbtshoot kbhardware kbprb kbnetwork_routerissues kbnetwork_generalbroadband kbexpertisebeginner KB928233

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