DHCPv4 and DHCPv6 client protocols manage network configuration information in different ways. The key difference is that with DHCPv4, the negotiation is for the lease of a single address and some options to go with it. With DHCPv6, the negotiation is over a batch of addresses and a batch of options.
For background information about the interaction between DHCPv4 client and a DHCP server, see About DHCP (Overview).
DHCPv6 communication between a DHCP client and a DHCP server begins with the client sending out a Solicit message, to locate servers. In response, all servers available for DHCP service send an Advertise message. The server message contains multiple IA_NA (Identity Association Non-Temporary Address) records plus other options (such as DNS server addresses) that the DHCP server can supply.
A DHCP client can request particular addresses (and multiples of them) by setting up its own IA_NA/IAADDR records in its Request message. A client typically requests specific addresses if it has old addresses recorded and it would be most efficient for the DHCP server to provide the same ones. Regardless of the client's actions (even if it requests no addresses at all), the DHCP server can supply any number of addresses to the DHCP client for a single DHCPv6 transaction.
The typical message dialogue that takes place between the DHCP clients and DHCP servers is as follows:
A DHCP client sends a Solicit message to locate DHCP servers.
DHCP servers send an Advertise message to indicate they are available for DHCP service.
A DHCP client sends a Request message to request configuration parameters, including IP addresses, from DHCP servers with the greatest preference values.
DHCP server preference values are set by the administrator and extend from 0, at the lowest end, to 255 at the highest.
The DHCP server sends a Reply message that contains the address leases and configuration data.
If the preference value in the Advertise message is 255, the DHCPv6 client immediately selects that DHCP server. If the most preferred server does not respond or fails to give a successful Reply message in answer to the Request message, then the client continues looking for less-preferred DHCP servers (in order) until it has no more Advertise messages. At that point, the client starts over by again sending Solicit messages.
The chosen DHCP server sends a Reply message containing assigned addresses and configuration parameters in response to a Solicit or Request message.