Duplicate Address Detection (DAD)

Duplicate Address Detection (DAD) allows hosts to check if other devices on the local link are using the same IPv6 address.

Any time an IPv6-enabled interface initializes (no shutdown command), or an IPv6 address is configured on an interface (by any method: manual, SLAAC, etc.), it performs DAD.

DAD uses two messages you learned earlier: NS and NA.

The host will send an NS to its own IPv6 address. If it doesn't get a reply, it knows the address is unique.

If it gets a reply, it means another host on the network is already using the address.