Cisco Certified Network Associate (CCNA) Practice Exam

In the EUI-64 method, what happens to the seventh bit of the first byte in the MAC address?

1. It is inverted

2. It is removed

3. It is duplicated

4. It is set to zero

The correct answer is: It is inverted

In the EUI-64 format, the seventh bit of the first byte in the MAC address is inverted to generate the corresponding IPv6 interface identifier. This transformation is part of the process to distinguish between globally unique addresses and locally administered addresses. The first byte of the MAC address is divided into two parts: the Organizationally Unique Identifier (OUI), which identifies the manufacturer, and the individual device identifier. The seventh bit, also known as the universal/local (U/L) bit, indicates whether the address is universally administered or locally administered. By inverting this bit, the system effectively signals that the IPv6 address will be based on the MAC address as a globally unique identifier, unless it is prefixed with a locally administered bit. This consistent manipulation of the seventh bit ensures that when a device generates its IPv6 address from a MAC address, the address can be consistently categorized and recognized across the network. Hence, inverting the seventh bit is a crucial step in creating a valid EUI-64 format address from a MAC address.