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.

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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.

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Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base: 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.

By setting this registry key to 1, Windows Vista will first try to obtain an IP address by using the BROADCAST flag in DHCP Discover packets. If that fails, it will try to obtain an IP address without using the BROADCAST flag in DHCP Discover packets.

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If you know the router or the non-Microsoft DHCP server does not support the DHCP BROADCAST flag, you can set the following registry entry as follows instead of using the DhcpConnEnableBcastFlagToggle registry key. 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.

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