Within the Source Address and Destination Address fields of the Ethernet II and IEEE 802.3
frame formats, special bits are defined, as Figure shows.




The Individual/Group Bit

The Individual/Group (I/G) bit is used to indicate whether the destination address is a unicast (individual) or multicast (group) address. For a unicast address, the I/G bit is set to 0. For a multicast address, the I/G bit is set to 1. The broadcast address is a special case of multicast, and its I/G bit is set to 1. The I/G bit is also known as the multicast bit.

The Universal/Locally Administered Bit
The Universal/Locally (U/L) Administered bit is used to indicate whether the IEEE allocated the address. For a universal address allocated by the IEEE, the U/L bit is set to 0. Universal addresses are guaranteed to be universally unique because network adapter manufacturers obtain universally unique vendor identifiers from the IEEE and assign unique 3-byte serial numbers to each network adapter. The 6-byte physical address of a network adapter, as programmed into the adapter during the manufacturing process, is a universally administered address.

For a locally administered address, the U/L bit is set to 1. Some network adapters allow you to override the network adapter’s physical address and specify a new physical address. In this case, the new address must have the U/L bit set to 1 to indicate that it is locally administered. The U/L bit is significant only for unicast addresses (the I/G bit is set to 0). When the I/G bit is set to 1, this bit does not imply either a locally or a universally administered address. The U/L bit is relevant for both the Source Address and Destination Address.

Routing Information Indicator Bit
The Routing Information Indicator bit, the low-order bit of the first byte of the source address, indicates whether MAC-level routing information is present. This bit is meaningful only for Token Ring addresses. Token Ring has a MAC-level routing mechanism known as Token Ring source routing. Even though this bit is meaningless for Ethernet addresses, it is still reserved and set to 0 to prevent problems when employing a translating bridge or Layer 2 switch between an Ethernet segment and a Token Ring ring.

For example, suppose the Routing Information Indicator bit is not reserved at the value of 0 for Ethernet addresses, and this bit is set to 1 through a universal or locally administered address. Then, when the address is translated to a Token Ring address, the Routing Information Indicator bit remains set to 1 even though there is no source routing information present, which can cause the Token Ring node to drop the frame.