Q & A on Identifying a DHCP client Q: I need you again for something I read about DHCP in your book. I do not know if you have already read my mail of yesterday but I wanted to ask you something else (again): If a client contacts a DHCP sever, it sends an identifier which is in general its MAC address, but do you know what else it can be? I suppose that it could be something else which permits to identify the machine physically but I am not sure and I can't imagine what exactly. A: The DHCP client by default uses its MAC address as its identifier. The DHCP client can send any arbitrary bytes in the 'client identifier' option, which the server then uses as an opaque or uninterpreted data value to identify the client. For example, the DHCP client could send a system ID number (perhaps read from an onboard PROM), or a system name entered by the user. The advantage to using a system ID number or system name as the 'client identifier' is that the computer will retain its identity even if the NIC is changed for some reason.