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