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DHCP client alternate configuration

Updated: January 21, 2005

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

DHCP client alternate configuration

With DHCP client alternate configuration, you can easily move a computer between two or more networks, one configured with static IP addresses and one or more configured with DHCP. Alternate configuration provides simplified computer migration (for example, a laptop) between networks without requiring that you reconfigure network adapter parameters such as IP address, subnet mask, default gateway, preferred and alternate Domain Name Service (DNS) servers, and Windows Internet Name Service (WINS) servers.

Configuration options

When you configure TCP/IP properties for a local area network connection, you have the following options:

  • Static IP address configuration

    When you clickUse the following IP address, you can provide values for static IP address settings, such as IP address, subnet mask, default gateway, preferred and alternate DNS servers, and WINS servers. However, if you click Obtain an IP address automatically to change the configuration of your network adapter to a DHCP client configuration, all static IP address settings are lost. Additionally, if you move the computer and configure it for another network, when you return to the original network you will need to reconfigure the computer with the original static IP address settings.

  • Dynamic IP address configuration without alternate configuration

    When you click Obtain an IP address automatically, your computer acts as a DHCP client and obtains IP address, subnet mask, and other configuration parameters from your network DHCP server. If the DHCP server is unavailable, the network adapter is configured using IP autoconfiguration.

  • Dynamic IP address configuration with alternate configuration

    When you click Obtain an IP address automatically, click the Alternate Configuration tab, and type an alternate configuration, you can move your computer between one statically configured network (such as a home network) and one or more dynamically configured networks (such as a corporate network) without changing any settings. If the DHCP server is unavailable (for example, when your computer is connected to your home network), the network adapter is automatically configured with your alternate configuration, and the computer functions correctly on the network. When you move the computer back to the dynamically configured network and the DHCP server is available, the network adapter is automatically configured with the dynamic configuration assigned by the DHCP server. The alternate configuration is used only when the DHCP client cannot locate a DHCP server.

DHCP server discover attempts

If you use DHCP without an alternate configuration and the DHCP client cannot locate a DHCP server, IP autoconfiguration is used to configure the network adapter. The DHCP client continues to attempt to discover a DHCP server on the network every five minutes. If a DHCP server is found, the network adapter is assigned a valid DHCP IP address lease.

If you use DHCP with an alternate configuration, and the DHCP client cannot locate a DHCP server, the alternate configuration is used to configure the network adapter. Normally, there are no additional discovery attempts. However, a DHCP server discovery attempt will occur if:

  • The network adapter is disabled and then enabled again.

  • Media (such as network cabling) is disconnected and then reconnected.

  • The TCP/IP settings for the adapter are changed, and DHCP remains enabled after these changes.

If a DHCP server is found, the network adapter is assigned a valid DHCP IP address lease.

Notes

  • It is possible that a static alternate configuration could conflict with the configuration of another computer on a network. For example, a client using alternate configuration might have the same IP address as another computer on the network. If this occurs, Address Resolution Protocol (ARP) detects the conflict, and the network adapter of the computer configured with the alternate configuration is automatically set to 0.0.0.0. No additional attempts are made to discover a DHCP server, obtain a lease, or use the static alternate configuration.

  • When switching from an alternate configuration to a configuration using IP autoconfiguration, the network adapter is configured with IP autoconfiguration settings (169.254.x.x), and a DHCP server discovery attempt is initiated at the same time. If the discovery is successful, the adapter is assigned a valid DHCP lease.

  • For more information, see Use DHCP client alternate configuration.

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