Identification of clients and servers plays very important role in process of DHCP.
Previous version (DHCP for IPv4) used MAC address of network interface as an unique client identificator. There are two new client identificators defined in DHCPv6
DUID is automatically generated on the first boot of operating system and it is generated according to link-layer or link-layer + boot time. The second option is usually used as default in most of the implementations. This cause problem, because network administrator is not able to determine DUID of newly added clients before the first connection and it changes with every reinstall of operating system. Each client or server has just one its own DUID.
IA is typically assigned to a configuration information set of one interface, equipped with a unique identifier (IAID).
The client is uniquely identified by its DUID, the interface within the client are then differentiated by IA.