<P> Addresses can either be universally administered addresses (UAA) or locally administered addresses (LAA). A universally administered address is uniquely assigned to a device by its manufacturer . The first three octets (in transmission order) identify the organization that issued the identifier and are known as the organizationally unique identifier (OUI). The remainder of the address (three octets for EUI - 48 or five for EUI - 64) are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness . A locally administered address is assigned to a device by a network administrator, overriding the burned - in address . </P> <P> Universally administered and locally administered addresses are distinguished by setting the second - least - significant bit of the first octet of the address . This bit is also referred to as the U / L bit, short for Universal / Local, which identifies how the address is administered . If the bit is 0, the address is universally administered . If it is 1, the address is locally administered . In the example address 06 - 00 - 00 - 00 - 00 - 00 the first octet is 06 (hex), the binary form of which is 00000110, where the second - least - significant bit is 1 . Therefore, it is a locally administered address . Another example that uses locally administered addresses is the DECnet protocol . The MAC address of the Ethernet interface is changed by the DECnet software to be AA - 00 - 04 - 00 - XX - YY where XX - YY reflects the DECnet network address xx. yy of the host . This eliminates the need for an address resolution protocol since the MAC address for any DECnet host can be simply determined . </P> <P> When the least significant bit of an address's first octet is 0 (zero), the frame is meant to reach only one receiving NIC . This type of transmission is called unicast . A unicast frame is transmitted to all nodes within the collision domain . In a modern wired setting the collision domain usually is the length of the Ethernet cable between two network cards . In a wireless setting, the collision domain is all receivers that can detect a given wireless signal . If a switch does not know which port leads to a given MAC address, the switch will forward a unicast frame to all of its ports (except the originating port), an action known as unicast flood . Only the node with the matching hardware MAC address will accept the frame; network frames with non-matching MAC - addresses are ignored, unless the device is in promiscuous mode . </P> <P> If the least significant bit of the first octet is set to 1, the frame will still be sent only once; however, NICs will choose to accept it based on criteria other than the matching of a MAC address: for example, based on a configurable list of accepted multicast MAC addresses . This is called multicast addressing . </P>

Source macs in an ethernet frame will always be unicast addresses