- SLAAC - Stateless Address Autoconfiguration
An IPv6 host connecting to the LAN should send an ICMPv6 Router Solicitation (RS) packet destined to the multicast IPv6 group FF02::2 (only local Routers will process the packet).
Upon reception of this packet, the local Router replies with an ICMPv6 Router Advertisement (RA). This packet includes among other information, the Network Prefix and Default Gateway for the subnet.
Thanks to this packet, the IPv6 hosts knows what is the Network Prefix and Default Gateway (DG) it should use. Actually, the DG is not part of the payload of the RA but the IPv6 host gets that info from the IPv6 Source address of the RA packet.
Finally, the host portion of the IPv6 address is derived by the IPv6 Host itself based on its own MAC address, commonly using a method called EUI-64 (I won't dig into this to keeps the explanation simple, just think on this as the way the Host creates its own IPv6 host portion of the addressing).
In conclusion, the IPv6 host can get its full IPv6 address (Network Prefix + Host portion) and Default Gateway without the need of a DHCPv6 Server.
SLAAC does not provide DNS information so the IPv6 host requires DHCPv6 if you want it to get the DNS IPv6 address automatically.
Note that DHCPv6 does not provide Default Gateway information so you still require the IPv6 host receive the RA packet from the Router.
IPv6 hosts gets:
- Network prefix information from the DHCPv6 Server.
- Host portion of the IPv6 addressing deriving it based on its own Mac Address (EUI-64 method).
- Default Gateway via the RA packet.
- DNS information from the DHCPv6 Server.
IPv6 hosts gets:
- Network prefix information from the DHCPv6 Server.
- Host portion of the IPv6 addressing from the DHCPv6 Server. Therefore stateful as the DHCPv6 server keeps a record of these.
- Default Gateway via the RA packet.
- DNS information from the DHCPv6 Server.
This document may also be of help:
Troubleshooting IPv6 dynamic address assignment between Cisco and Windows
