What are unicast and multicast addresses?

There are also two different types of Ethernet addresses - unicast and multicast.

If the least significant bit of the most significant octet of an address is set to 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, which typically ends at the nearest network switch or router. A switch will forward a unicast frame through all of its ports (except for the port that originated the frame) if the switch has no knowledge of which port leads to that MAC address, or just to the proper port if it does have the knowledge. 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 a promiscuous mode.

If the least significant bit of the most significant address 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.