A Dynamic Host Configuration Protocol (DHCP) client will normally use the media access control address of its network interface card (NIC) to identify itself to a DHCP server. Windows NT DHCP clients can also have client identifiers manually configured.

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When DHCP clients request and renew IP addresses, they normally send their hardware type and media access control address to a DHCP Server as a unique identifier. This allows the DHCP server to track the allocation of addresses and allows administrators to reserve addresses for specific interfaces. This client identifier is sent as option 61 in the client's DHCP Discover and Request packets as described in RFC2132, "DHCP Options and BOOTP Vendor Extensions." In general, this identifier will be the media access control address of the network interface card requesting the address, but this is not required.

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

Implementing a user-defined DHCP client identifier requires Windows NT 4.0 SP2 or later. To manually configure a DHCP client identifier, perform the following 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 Pro network adapter the <Adapter Name> is Epro so you might have an entry similar to Epro1. In Windows 2000 the location is: HKLM\System\CurrentControlSet\Services\TcpIp\Parameters\Interfaces\NICGUID
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.

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Creating a DHCP Server Reservation

A DHCP server can use a custom DHCP client identifier to reserve an IP address for a specific network interface card. If the NIC is replaced, the new NIC will receive the same IP address from the DHCP server. When a Windows NT DHCP client sends a client identifier to a DHCP server, it sends the identifier as 4 bytes, or 8 hexadecimal characters. The identifier is sent in groups of two hexadecimal characters, with these groups of two sent in 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 Server

12345678                      78563412
123456                        56341200
1234                          34120000
1234567                       67452301
12345                         45230100
123                           23010000
A18F42                        428FA100
CF432                         32F40C00
C32D1BE                       BED1320C
				

Refer to the online Help in DHCP Manager under the section "Managing Client Reservations" and pages 206-207 of the Windows NT 4.0 Server Networking Guide.

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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 or on a subscription basis (for more information contact NIC@NIC.DDN.MIL (mailto:NIC@NIC.DDN.MIL)). Online copies are available via FTP or Kermit from NIC.DDN.MIL as rfc/rfc####.txt or rfc/rfc####.PS (#### is the RFC number without leading zeroes).

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