If the U/L bit of a MAC address is 1, what does it signify?

When the U/L (Universal/Local) bit of a MAC address is set to 1, it signifies that the address is a local address. MAC addresses are typically 48 bits in length, with the first 24 bits representing the Organizationally Unique Identifier (OUI) and the last 24 bits assigned by the manufacturer. The U/L bit is the second least significant bit of the first byte of the MAC address.

By setting the U/L bit to 1, manufacturers indicate that the address is administered locally—meaning it can be assigned by the user or organization—and is not globally unique. This allows for the creation of custom or locally significant MAC addresses. In contrast, if the U/L bit is set to 0, it indicates a universally administered address, which is supposed to be globally unique and assigned by the manufacturer.

The U/L bit thus plays a key role in distinguishing between locally defined addresses and those standardized by manufacturers, making it essential for managing network communications effectively and securely.