Cisco Certified Network Associate (CCNA) Practice Exam

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When using EUI-64, how many bits are used for the Interface ID in an IPv6 address?

48 bits
64 bits
128 bits
32 bits

The correct answer is: 64 bits

The Interface ID in an IPv6 address, when using EUI-64, is indeed 64 bits long. In the context of IPv6 addressing, the overall address is 128 bits, comprised of the first 64 bits for the network prefix and the last 64 bits for the Interface ID. EUI-64 format specifically derives the Interface ID from the MAC address of the network interface. This is achieved by taking the 48-bit MAC address, inserting a fixed 16-bit value in the middle, and inverting the seventh bit of the first byte of the MAC address. The resulting structure ends up being exactly 64 bits for the Interface ID. Thus, the answer is 64 bits, which aligns with the design of EUI-64 to standardize the way hosts generate unique identifiers for network interfaces in IPv6.