I found this explanation of Client-Id in DHCP FAQ (http://www.dhcp-handbook.com/dhcp_faq.html#wiaci)
What is termed the Client ID for the purposes of the DHCP protocol is whatever is used by the protocol to identify the client computer. By default, DHCP implementations typically employ the client’s MAC address for this purpose, but the DHCP protocol allows other options. Some DHCP implementations have a setup option to specify the client ID you want. One alternative to the MAC address is simply a character string of your choice. In any case, in order for DHCP to function, you must be certain that no other client is using the client ID you choose, and you must be sure the DHCP server will accept it.
How to use client-id in MT’s DHCP lease setup? If I left this feild blank it accepts client by MAC only but it shows client-id in active-client-id which is very similar to MAC. Is there a way I can use custom client-id’s for each client? My clients are all on Windows platform.