When you reserve an IP address for a DHCP client, you provide the MAC address of the client's NIC.

It is possible to use a custom identifier, which is sent as option 61 in the client's DHCP Discover and Request packet.

The DhcpClientIdentifier is a REG_DWORD value that is located at:

Windows NT 4.0 SP2+

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\<Adapter Name>'X'\Parameters\Tcpip

where <Adapter Name> is the NIC driver name and 'X' is the number of the NIC.

Windows 2000

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\TcpIp\Parameters\Interfaces\<NIC GUID>

where <NIC GUID> is the GUID of the NIC.

The valid range of data is 0x0 - 0xFFFFFFFF. The custom identifier is send as 4 bytes, 8 hexadecimal character, in groups of 2 hexadecimal characters, with the groups being sent in reverse order. If the custom identifier is less than 8 hexadeciaml characters, it is zero padded at the end. Examples:

Custom Client                 Client Reservation                              Identifier                    on DHCP Server                              12345678                      78563412                              123456                        56341200                              1234                          34120000                              1234567                       67452301                              12345                         45230100                              123                           23010000                              A18F42                        428FA100                              CF432                         32F40C00                              C32D1BE                       BED1320C