How is an Ethernet MAC address constructed?

How is an Ethernet MAC address constructed?

• The first 3 bytes represent the network upon which the host resides, and the last 3 bytes uniquely identify the host.
• The first 3 bytes represent the vendor OUI, and the last 3 bytes are assigned uniquely by the vendor
• The first 3 bytes are uniquely assigned by LANA, and the last 3 bytes represent the serial number of the host
• The first 3 byles represent the vendor OUI, and the last 3 byles are assigned randorily by the operating system.