What is IPv6 Autoconfiguration?


IPv6 addresses are significantly uglier than IPv4 addresses, so one of the goals of the IPv6 standards is to reduce the number of times network managers and end users enter these addresses. In addition, the highly successful results which have come from deployments in enterprise networks using DHCP and PPP have proven the value of dynamic assignment of IP addresses.

Two Types of Automatic Configuration

IPv6 offers two types of autoconfiguration; this is one of the key benefits of IPv6. Autoconfiguration offers true plug-and-play connectivity in the same way that DHCP provides a similar autoconfiguration for IPv4.



Stateful Autoconfiguration

The two types of autoconfiguration are "stateless" and "stateful." Stateful autoconfiguration is the IPv6 equivalent of DHCP. A new protocol, called DHCPv6 (and based closely on DHCP), is used to pass out addressing and service information in the same way that DHCP is used in IPv4. This is called "stateful" because the DHCP server and the client must both maintain state information to keep addresses from conflicting, to handle leases, and to renew addresses over time.

Our network does not use DHCPv6. The DHCPv6 protocol is not yet standardized, although there are several drafts available, including "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)" (by J. Bound and C. Perkins) "Extensions for DHCPv6" (by C. Perkins) which are expected to move to proposed standard status shortly.

Stateless Autoconfiguration

The other type of autoconfiguration in IPv6 is Stateless Autoconfiguration. With Stateless Autoconfiguration, a host gains an address via an interface automatically "leasing" an address and does not require the establishment of an server to delve out address space. Stateless autoconfiguration allows a host to propose an address which will probably be unique (based on the network prefix and its Ethernet MAC address) and propose its use on the network. Because no server has to approve the use of the address, or pass it out, stateless autoconfiguration is simpler. This is the default mode of operation for most IPv6 systems, including servers.

You can see how our Windows 2000 system has handled its IPv6 configuration by opening up a Command window. The command IPV6 IF will show you the interfaces for IPv6. Of these, interface 4 is the local area network connection. Notice that it has "autoconfigured" its address, and the low order 48 bits of the address look surprisingly like a MAC address. If you give the IPV6 RT command, you can see that our Windows 2000 system has also automatically discovered its router address.

To see how simple it is to configure IPv6, go to the "Start->Settings->Network&Dial-Up Connections->Local Area Connection" menu and click "Properties." Notice when you select IPv6 that there are no properties---there is nothing to set up!

Stateless autoconfiguration is described in RFC2462 ("IPv6 Stateless Address Autoconfiguration") with some proposed extensions in the Internet draft "Privacy Extensions for Stateless Address Autoconfiguration in IPv6."

One of the advantages of DHCP is that it does more than handle IP address allocation. For example, DHCP can be used to let end systems discover their DNS servers. Thus, large networks will likely use a combination of stateless autoconfiguration to discover their address, and DHCPv6 servers to pass out other information, such as DNS servers, with a third mechanism for the system to register its name in the DNS.