The MAC (Media Access Control) address is hardware address of a device connected to a network. The MAC address is a computer's unique hardware number for the network interface card (NIC). Note that the MAC address belongs to the NIC rather than to the computer itself. The MAC address contains 48 bits.

While the MAC address should be unique, it is possible to overwrite it manually by the Network Administrator, or by an intruder (MAC spoofing). IP addresses are assigned to MAC addresses usually using Address Resolution Protocol (ARP). Thus, IP addresses on a network should be unique. However, if two different computers have the same IP address, it is possible that they have the same MAC address erroneously. The most common cause for this is system administrators who might neglect to configure a specific setting in their virtual machines (such as VMware). If this is the case, the TCP/IP backbone required by DCOM will not work properly and thus OPC communication will likely fail.