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Media Access Control Layer

The MAC layer is responsible for transmitting LLC data between two stations over the physical media. This layer specifies the MAC address and frame format. Each frame will contain a destination and source MAC address. This address is 48 bits long.

The most significant bit of the MAC address is the Individual/Group Address Bit (I/G Address Bit). If this bit is set to a zero, this is an individual address, which means that this frame is destined for a single station. This is also called unicast traffic. If the I/G Address Bit is set to 1, this is a group address meaning that the frame is destined for more than one station. There are two types of group addresses. Group addresses are either functional addresses or broadcast addresses. If all the bits of the MAC address are set to one, this is a broadcast address. If not, this is a functional or multicast address.

The second most significant bit of the MAC address is the Universally or Locally Administered Address Bit (U/L Address Bit). If this bit is set to 0, the address is administered universally. The IEEE does this by assigning blocks of MAC addresses to the various Ethernet vendors. In this scheme, the first 24 bits of the MAC address are called the Organizational Unique Identifier (OUI). OUIs are apportioned out to Ethernet vendors who then administer the last 24 bits to guarantee globally unique addresses. Individuals are not required to use the universally administered MAC address, also called the burned-in address. They locally administer the MAC address by setting the U/L Address Bit to 1. Locally administered MAC addresses must be unique.

Key Concept

MAC addresses must be unique for each station and support unicast, multicast, and broadcast traffic.

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