DOCUMENT:Q161430  08-AUG-2001  [winnt]
TITLE   :DHCP: Detecting and Flagging Duplicate IP Addresses
PRODUCT :Microsoft Windows NT
PROD/VER:winnt:4.0
OPER/SYS:
KEYWORDS:kbnetwork

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The information in this article applies to:

 - Microsoft Windows NT Workstation version 4.0 
 - Microsoft Windows NT Server version 4.0 
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SUMMARY
=======

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.

MORE INFORMATION
================

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

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Keywords          : kbnetwork 
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400
Version           : winnt:4.0

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