Custom DHCP Client Identifiers for Windows NT

This article was previously published under Q172408
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A Dynamic Host Configuration Protocol (DHCP) client will normally use themedia access control address of its network interface card (NIC) toidentify itself to a DHCP server. Windows NT DHCP clients can also haveclient identifiers manually configured.
When DHCP clients request and renew IP addresses, they normally send theirhardware type and media access control address to a DHCP Server as a uniqueidentifier. This allows the DHCP server to track the allocation ofaddresses and allows administrators to reserve addresses for specificinterfaces. This client identifier is sent as option 61 in the client'sDHCP Discover and Request packets as described in RFC2132, "DHCP Optionsand BOOTP Vendor Extensions." In general, this identifier will be the mediaaccess control address of the network interface card requesting theaddress, but this is not required.

Most implementations of DHCP are simplified by allowing the Windows NTclient to automatically specify their media access control address, but itis possible in Windows NT 4.0 to manually enter a client identifier. Thisallows an administrator to reserve an IP address for a DHCP client so theclient will receive the same IP address even if its network card isreplaced.

Implementing a user-defined DHCP client identifier requires Windows NT 4.0SP2 or later. To manually configure a DHCP client identifier, perform thefollowing steps:

WARNING: Using Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT to correct them. Microsoft cannot guarantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk.
  1. Start Registry Editor (Regedt32.exe) and select the following subkey:
    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ <Adapter Name>'X'\Parameters\Tcpip.							

    where <Adapter Name> is the network adapter driver name and 'X' is the number of the adapter. For example, for an Intel Etherexpress Pronetwork adapter the <Adapter Name> is Epro so you might have an entrysimilar to Epro1.

    In Windows 2000 the location is:
  2. Click Add value from the Edit menu, and type the following information:
          Value Name: DhcpClientIdentifier      Value Type: REG_DWORD      Value Range: 0x0 - 0xFFFFFFFF      Value Default: 0x0						
  3. Click OK, exit Registry Editor, and restart the system for the changes to take effect.

Creating a DHCP Server Reservation

A DHCP server can use a custom DHCP client identifier to reserve an IPaddress for a specific network interface card. If the NIC is replaced, thenew NIC will receive the same IP address from the DHCP server. When aWindows NT DHCP client sends a client identifier to a DHCP server, it sendsthe identifier as 4 bytes, or 8 hexadecimal characters. The identifier issent in groups of two hexadecimal characters, with these groups of two sentin reverse order. If the identifier is less than 8 hexadecimal characters,then zeros are padded at the end of the identifier. For example:
Custom Client Identifier      Client Reservation on DHCP Server12345678                      78563412123456                        563412001234                          341200001234567                       6745230112345                         45230100123                           23010000A18F42                        428FA100CF432                         32F40C00C32D1BE                       BED1320C				

Refer to the online Help in DHCP Manager under the section "Managing ClientReservations" and pages 206-207 of the Windows NT 4.0 Server NetworkingGuide.
For more information on DHCP reservations, see RFC1700 and RFC2131.

RFCs may be obtained through the Internet as follows:

Paper copies of all RFCs are available from the NIC, either individually oron a subscription basis (for more information contact NIC@NIC.DDN.MIL).Online copies are available via FTP or Kermit from NIC.DDN.MIL asrfc/rfc####.txt or rfc/rfc####.PS (#### is the RFC number without leadingzeroes).

Article ID: 172408 - Last Review: 12/04/2015 17:36:26 - Revision: 2.2

Microsoft Windows 2000 Server, Microsoft Windows 2000 Advanced Server, Microsoft Windows 2000 Professional Edition, Microsoft Windows NT Workstation 4.0 Developer Edition, Microsoft Windows NT Server 4.0 Standard Edition

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