System Administration Guide: IP Services  MAC Address and Client
ID  DHCPv4 uses the MAC address and an optional Client ID to
identify the client for purposes of assigning an address. Each
time the same client arrives on the network, it gets the same
address, if possible.  DHCPv6 uses basically the same scheme,
but makes the Client ID mandatory and imposes structure on it.
The Client ID in DHCPv6 consists of two parts: a DHCP Unique
Identifier (DUID) and an Identity Association Identifier (IAID).
The DUID identifies the client system (rather than just an
interface, as in DHCPv4), and the IAID identifies the interface
on that system.  As described in RFC 3315, an identity
association is the means used for a server and a client to
identify, group, and manage a set of related IPv6 addresses. A
client must associate at least one distinct IA with each of its
network interfaces, and then uses the assigned IAs to obtain
configuration information from a server for that interface. For
additional information about IAs,