Similar to IPv4, IPv6 global unicast addresses can be configured manually or dynamically. However, there are two methods in which IPv6 global unicast addresses can be assigned dynamically:
- Stateless Address Autoconfiguration (SLAAC), as shown in the figure
- Dynamic Host Configuration Protocol for IPv6 (Stateful DHCPv6)
SLAAC is a method in which a device can obtain an IPv6 global unicast address without the services of a DHCPv6 server. At the core of SLAAC is ICMPv6. ICMPv6 is similar to ICMPv4 but includes additional functionality and is a much more robust protocol. SLAAC uses ICMPv6 Router Solicitation and Router Advertisement messages to provide addressing and other configuration information that would normally be provided by a DHCP server:
- Router Solicitation (RS) message - When a client is configured to obtain its addressing information automatically using SLAAC, the client sends an RS message to the router. The RS message is sent to the IPv6 all-routers multicast address FF02::2.
- Router Advertisement (RA) message - RA messages are sent by routers to provide addressing information to clients configured to obtain their IPv6 addresses automatically. The RA message includes the prefix and prefix length of the local segment. A client uses this information to create its own IPv6 global unicast address. A router sends an RA message periodically, or in response to an RS message. By default, Cisco routers send RA messages every 200 seconds. RA messages are always sent to the IPv6 all-nodes multicast address FF02::1.
As the name indicates, SLAAC is stateless. A stateless service means there is no server that maintains network address information. Unlike DHCP, there is no SLAAC server that knows which IPv6 addresses are being used and which ones are available.