Which IPv6 Neighbor Discovery feature detects whether an address is already in use on the local link?

• A. Duplicate Address Detection
• B. Neighbor Reachability
• C. Router Discovery
• D. Router Solicitation

Answer: (A)

Duplicate Address Detection checks whether an IPv6 address is already in use on the local link. When a device configures a new IPv6 address, it marks it tentative and runs DAD by sending a Neighbor Solicitation for that address to the solicited-node multicast address, with the source address set to the unspecified address. If another device on the link owns the address, it replies with a Neighbor Advertisement, signaling a conflict and that the address is in use. If no such response is received after the retry period, the address is considered unique and can be assigned to the interface. This prevents two devices from sharing the same IPv6 address on the same network segment. The other features are for discovering or verifying routers and neighbor reachability, not for checking address uniqueness.