Stateless Address Autoconfiguration

Stateless address autoconfiguration requires no manual configuration of hosts, minimal configuration of routers, and no additional servers. The primary interface (lanX:0) is automatically assigned a link-local address by the system when the interface is configured. This allows each IPv6 interface to have at least one source address that can be used by Neighbor Discovery. Therefore, it is not advisable to assign other addresses to the primary interface besides the link-local address. See RFC 2373 “IP Version 6 Addressing Architecture” for details.

Link-Local Address Assigned Automatically

A link-local address is formed by prepending the well-known link-local prefix FE80::/10 to the interface identifier which is typically 64 bits long and based on EUI-64 identifiers. Link-local addresses are sufficient for allowing communication among IPv6 hosts attached to the same link.

Figure 6-3 “Primary Interface Address Autoconfiguration” shows the Primary Interface Autoconfiguration steps performed after using the ifconfig command, which is as follows:

ifconfig lan0 inet6 up

Figure 6-3 Primary Interface Address Autoconfiguration

If you mark an interface “up” without assigning a primary address, the system derives a link-local address by performing the following 4 steps:

Taking the LAN card’s 48-bit link-level address (“MAC address” 8:0:9:78:f3:39)
0000 1000 0000 0000 0000 1001 0111 1000 1111 0011 0011 1001
and putting it into an EUI-64 identifier by:
Putting two bytes (0xffee) into the middle (bit 24) of the 48-bit link-level address 8:0:9:ff:fe:78:f3:39;
```
0000 1000 0000 0000 0000 1001 1111 1111 1111 1110 0111 1000 1111 0011 0011 1001
```
and flipping the Universal/local bit (as described in RFC 2373) to form a 64-bit EIU-64 interface identifier a:0:9:ff:fe:78:f3:39
```
0000 1010 0000 0000 0000 1001 1111 1111 1111 1110 0111 1000 1111 0011 0011 1001
```
Prepending the well-known prefix fe80::/10
Forming a 128-bit link-local unicast address for the primary interface fe80::a00:9ff:fe78:f339

View the configuration by typing

ifconfig lan0 inet6

lan0: flags=4800841<UP,RUNNING,MULTICAST,ONLINK>
inet6 fe80::a00:9ff:fe78:f339 prefix 10

Secondary Interface Autoconfiguration

If an IPv6 router on the network advertises network prefixes in router advertisements , IPv6 derives a second IPv6 address based on the interface identifier. IPv6 assigns this address to a secondary interface for the network interface. The host adds the router as one of its default gateways. In general, there are as many secondary interfaces configured as there are prefixes advertised by the router.

Figure 6-4 “Secondary Interface Autoconfiguration From an IPv6 Router” shows a general example of Secondary Interface Autoconfiguration.

Figure 6-4 Secondary Interface Autoconfiguration From an IPv6 Router

Primary interface comes up with the link-local address autoconfigured.
Host multicasts Router Solicitation.
IPv6 Router sends Router Advertisement to host.
Host autoconfigures secondary interface (lan0:1) by prepending prefix (3ffe:2000:0:13::/64) sent by router to interface identifier ( a00:9ff:fe78:f339). Refer to RFC 2461 “Neighbor Discovery for IP Version 6 (IPv6)” for details.

Manual Configuration and Router Advertisements

Note that even if a primary interface is manually configured, if the host receives prefixes from router advertisements, then secondary interfaces are autoconfigured. In this case, the addresses on the secondary interfaces are derived from the interface ID portion of the manually specified primary interface address.

Manual Configuration Overwriting Autoconfiguration

Manual configuration can overwrite autoconfiguration. When a secondary interface is configured with a manually assigned address, and if the user chooses an interface index number that has been used for an already autoconfigured secondary interface, the manual configuration overwrites the autoconfiguration. When this happens, network connectivity through the overwritten autoconfigured IP address is temporarily lost. At a later time, when the host receives the next router advertisement, the host will bring up another secondary interface with a different IP index number, but with the same IP address, and network connectivity through that IP address is restored. Normally, a user can avoid this by checking used IP index numbers. However, there is always a possibility that address autoconfiguration due to router advertisement is happening concurrently while the user manually configures secondary interfaces.

Disabling Specific IPv6 Interfaces

To disable communication through a specific IP address on an autoconfigured secondary interface, that secondary interface must be marked down, not removed or overwritten with a different IP address. If that interface is removed or overwritten, the host will reconfigure another secondary interface with the same IP address when it receives the next router advertisement. Alternatively, the router can be configured to stop advertising the prefix that corresponds to the offending IP address.

Removing An Interface

A primary interface cannot be removed from the system until all secondary interfaces are removed. You can remove secondary interfaces from the system using the ifconfig inet6 command, as in the following example:

ifconfig lan1:1 inet6 ::

The primary interface (for example, lan1) can then be removed from the system with the ifconfig command, as in the following example:

ifconfig lan1 inet6 unplumb

A loopback interface does not have a hardware device associated with it. The name of the loopback interface is lo0. A loopback interface is automatically created by the system. You cannot delete it.