JSI Tip 2845. How can I keep the same DHCP client reservation, if the MAC address changes?

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_MACHINESYSTEMCurrentControlSetServices'X'ParametersTcpip

where is the NIC driver name and 'X' is the number of the NIC.

Windows 2000

HKEY_LOCAL_MACHINESystemCurrentControlSetServicesTcpIpParametersInterfaces

where 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 ReservationIdentifier                    on DHCP Server12345678                      78563412123456                        563412001234                          341200001234567                       6745230112345                         45230100123                           23010000A18F42                        428FA100CF432                         32F40C00C32D1BE                       BED1320C