2.1 Legacy Media Types
2.1.3 Ethernet addressing
How do stations identify each other? In Ethernet, an application can choose to address the entire group, a set of hosts, or a specific host within the scope of communication (the Ethernet segment). Speaking to the group requires a broadcast; contacting a set of individual stations requires a multicast; and addressing one end system requires a unicast. Most traffic in a network is unicast in nature, characterized as traffic from a specific station to another specific device. Some applications generate multicast traffic. Examples include multimedia services over LANs. These applications intend for more than one station to receive the traffic, but not necessarily all the stations.

Video conferencing applications frequently implement multicast addressing to specify a group of recipients. Networking protocols typically have a need to create broadcast traffic in certain instances. For example, IP creates broadcast packets for Address Resolution Protocol (ARP) requests. Routers often transmit routing updates as broadcasts. AppleTalk, Novell Internetwork Packet Exchange (IPX), and other Layer 3 protocols create broadcasts to perform name resolution and to carry out various other functions.

The Figure shows a simple Ethernet system with several devices attached. The Ethernet adapter card of each device has a 48-bit (6-octet) address built into it that uniquely identifies the station. This is called the Media Access Control (MAC) address, or the hardware address. All the devices in a LAN must have a unique MAC address. Devices express MAC addresses as hexadecimal values. Sometimes MAC address octets are separated by hyphens "-", sometimes by colons ":", and sometimes by periods ".". The three formats, 00-60-97-8F-4F-86, 00:60:97:8F:4F:86, and 0060.978F.4F86, all specify the same host.

To help ensure uniqueness, the first three octets indicate the vendor that manufactured the interface card. This is known as the Organizational Unique Identifier (OUI). Each manufacturer has a unique IEEE-assigned OUI value.

The last three octets of the MAC address amount to a host identifier for the device. The last three octets are assigned by the vendor. The combination of OUI and "host number" creates a unique address for that device. Each vendor is responsible to ensure that each of the Ethernet adapters that it manufactures has a unique combination of six octets.