The lower order 64 bits of the SLAAC link-local address are derived from which source using EUI-64 formatting?

• A. MAC address
• B. Random 64-bit value
• C. IP address
• D. Prefix

Answer: (D)

In SLAAC, a link-local IPv6 address uses the fe80::/64 prefix for the upper 64 bits. The lower 64 bits form the interface identifier, which, when using EUI-64 formatting, comes from the device’s MAC address. The MAC is expanded to 64 bits by inserting FF:FE in the middle and flipping the universal/local bit in the first octet, producing a unique interface identifier derived from the MAC. If privacy extensions are in use, that lower 64 bits could be a random value instead, but with standard EUI-64 SLAAC, the source is the MAC address. The prefix determines the upper half, not the source of the lower half.