Assignment of IPv6 addresses to a host can occur statically or dynamically. Static IPv6 address assignment involves manual configuration on the host’s configuration files. Dynamic IPv6 address assignment can be done via stateless or stateful methods. Stateless address assignment may result in a link-local or globally unique address. The following sections explain these three methods to assign IPv6 addresses:
- Manual configuration
- Stateless address autoconfiguration (SLAAC)
- Stateful configuration with DHCPv6
Manual Configuration
As with IPv4, devices such as routers, switches, servers, and firewalls should have IPv6 addresses configured manually.
SLAAC of Link-Local Address
The dynamic configuration of link-local IPv6 addresses is a stateless autoconfiguration method—that is, without DHCP. Hosts obtain their link-local addresses automatically as an interface is initialized. First, the host performs a duplicate address-detection process. The host joins the all-nodes multicast group to receive neighbor advertisements from other nodes. The neighbor advertisements include the subnet or prefix associated with the link. The host then sends a neighbor-solicitation message with the tentative IP address (interface identifier) as the target. If a host is already using the tentative IP address, that host replies with a neighbor advertisement. If the host receives no neighbor advertisement, the target IP address becomes the link-local address of the originating host. It uses the link-local prefix FE80::/10 (binary: 1111 1110 10). An alternative is to manually configure the link-local address.
SLAAC of Globally Unique IPv6 Address
RFC 4862 describes IPv6 stateless address autoconfiguration. With autoconfiguration of globally unique IP addresses, IPv6 hosts can use SLAAC—without DHCP—to acquire their own IP address information. This is done on a per-interface basis. As shown in Figure 2-9, after a host has autoconfigured a link-local address, it listens for router advertisement (RA) messages. These router messages contain the prefix address to be used for the network. The IPv6 address is then formed from the prefix plus the interface ID (which derives from the MAC address).
Figure 2-9 Stateless Autoconfiguration
Creating the globally unique IPv6 address using SLAAC involves the following steps:
Step 1. Router advertisement (RA) messages are sent by Router 1.
Step 2. The client learns the prefix from the RA message. In the case of Figure 2-9, the prefix is 2001:abcd:1234/64.
Step 3. The client identifier is created by splitting the local MAC address and adding FF:FE in the middle. Hence, in this example, the MAC address 0200:FE23:5A6B becomes 0200:FEFF:FE23:5A6B.
Step 4. The seventh bit is flipped (binary 00000010 becomes binary 0000000); thus, the identifier becomes 0000:FEFF:FE23:5A6B.
Step 5. The merging of the prefix and identifier becomes 2001:abcd:1234:0000:0000: FEFF:FE23:5A6B.
Step 6. The address is shortened to 2001:abcd:1234:: FEFF:FE23:5A6B.
Table 2-8 summarizes IPv6 address configuration schemes.
Table 2-8 IPv6 Address Autoconfiguration Scheme
| IPv6 Address Configuration Scheme | Description |
| Manual configuration | Used for routers, switches, servers, and firewalls. |
| SLAAC link-local | Host sends a Neighbor Solicitation message that includes the target IPv6 address that begins with FE80::. |
| SLAAC global unique | Combines the router prefix with the local MAC address. |
| DHCPv6 | Provides stateful address allocation. |