I believe what you're referring to is having your Macs automatically adding an Open Directory server by adding a DHCP option, which is possible. I don't recall the exact DHCP option. I don't remember if Apple got rid of this in 10.5 and above or not. I know it was something you could do to 10.4 clients. Just a "minor" security issue to let any DCHP server offer up an Open Directory server and it's managed preferences (MCX).
MAC - A media access control address (MAC address) is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used as a network address for most IEEE 802 network technologies, including Ethernet. Logically, MAC addresses are used in the media access control protocol sublayer of the OSI reference model.Mac (iMac, Mac OSx, etc)...