How is the U/L bit set for a locally administered address?

The U/L bit, also known as the Universal/Local bit, is part of the MAC address structure, which is a 48-bit identifier assigned to network interfaces for communications on the physical network segment. In the context of MAC addresses, if the U/L bit is set to 1, it designates the address as a locally administered address. This means that the MAC address has been manually configured by a network administrator rather than being assigned by the manufacturer.

When creating a MAC address for a device, the manufacturer typically sets the U/L bit to 0. In this state, the address is considered universal, meaning it is assigned according to the standards and registered with the IEEE. However, when a network administrator needs to override this default behavior and create a locally administered address, they set the U/L bit to 1.

This is crucial in scenarios where unique MAC addresses are necessary, such as in virtual environments or when replacing a defective network interface without changing the MAC address used by upper-layer protocols. By utilizing locally administered addresses, networks can maintain greater control and flexibility.

In summary, setting the U/L bit to 1 indicates that the address is locally administered, allowing administrators to create unique addresses as needed in their network environments.