Microsoft KB Archive/161430

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Knowledge Base

DHCP: Detecting and Flagging Duplicate IP Addresses

Article ID: 161430

Article Last Modified on 11/1/2006


  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 4.0 Standard Edition

This article was previously published under Q161430


Windows NT 4.0 Service Pack 2 (SP2) provides updated code to assist DHCP in detecting and flagging duplicate IP addresses. Functionality has been added both at the client and at the Windows NT DHCP server.


The following features were added to Windows NT 4.0 in SP2 to assist in detecting and working around duplicate IP addresses:

  • The DHCP server has a new configurable option to allow the administrator to specify duplicate address detection. To enable this feature, run the DHCP Administrator, select a DHCP server, and select Server->Properties. There is a new "Conflict Detection Attempts" listbox that allows the server to be configured to attempt to ping each IP address before issuing a lease for it.

NOTE: This functionality was added to aid in resolving duplicate IP address situations resulting from accidental mis-configurations. This feature was not intended as a mechanism to allow deployment of overlapping scopes and this type of deployment is not supported by Microsoft.

NOTE: The address conflict detection feature can limit the throughput of the DHCP server. The DHCP server must wait several seconds for each retry to timeout or respond before servicing clients. For improved performance, reduce the number of ping retries in the Server Properties dialog box.

The number of pings is configurable, and after sending each ping the server waits two seconds for a reply. The time required for a client to obtain a lease will be increased by (the number of pings selected) * 2, so this value should be chosen carefully. Normally, a setting of 1 or 2 should be adequate.

If the DHCP server receives a reply to any of the pings, it will mark the IP address as a "BAD_ADDRESS" and will attempt to lease the next address in the scope. After the offending duplicate address is removed from the network, the "BAD_ADDRESS" can be returned to the pool by deleting it from the scope Active Leases dialog box.

NOTE: The DHCP server only pings IP addresses that have not been previously leased successfully. If a client is being leased an IP address that it already had, or is requesting a renewal, the DHCP server will not issue any pings before sending the DHCP OFFER or DHCP ACK.

  • Windows NT 4.0 SP2 clients now have support for issuing DHCP DECLINE messages. After the client receives a lease from the DHCP server, it will send an ARP for the IP address it has been assigned. If any other client replies to that ARP, the address is already in use. In this case, the client will issue a DHCP DECLINE to the DHCP server. The Windows NT 4.0 SP2 server will flag the IP address as a "BAD_ADDRESS" and the client will go through the lease process again, receiving the next available address in the scope.

Additional query words: dhcp duplicate IP address decline conflict dupe

Keywords: kbnetwork KB161430