Windows 7 / Networking

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Understanding Address Autoconfiguration

On IPv4 networks, addresses can be assigned to hosts in three ways:

• Manually, using static address assignment
• Automatically, using Dynamic Host Configuration Protocol (DHCP) if a DHCP server is present on the subnet (or a DHCP relay agent is configured on the subnet)
• Automatically, using APIPA, which randomly assigns the host an address from the range 169.254.0.0 to 169.254.255.255 with subnet mask 255.255.0.0

On IPv6 networks, static addresses are generally assigned only to routers and (sometimes) servers, but hardly ever to client computers. Instead, IPv6 addresses are almost always assigned automatically using a process called address autoconfiguration. Address autoconfiguration can work in three ways: stateless, stateful, or both. Stateless address autoconfiguration is based on the receipt of ICMPv6 Router Advertisement messages. Stateful address autoconfiguration, on the other hand, uses DHCP for IPv6 (DHCPv6) to obtain address information and other configuration settings from a DHCPv6 server.

Note The DHCP Server service of Windows Server 2008 supports DHCPv6. The DHCP Server service of Windows Server 2003 does not support DHCPv6.

All IPv6 nodes (hosts and routers) automatically assign themselves link-local addresses (addresses having the address prefix FE80::/64); this is done for every interface (both physical and logical) on the node. (6to4 interfaces are an exception-they might not have link-local addresses automatically assigned.) These autoconfigured link-local addresses can be used only to reach neighboring nodes (nodes on the same link). When specifying one of these addresses as a destination address, you might need to specify the zone ID for the destination. In addition, link-local addresses are never registered in DNS servers.

Note Manual assignment of IPv6 addresses is generally needed only for IPv6 routers and for some servers. You can configure a computer running Windows 7 with multiple interfaces to be used as a router. For more information on configuring IPv6 routers, see the Cable Guy article titled "Manual Configuration for IPv6" at http://technet.microsoft.com/en-us/library/bb878102.aspx. For a description of the IPv6 routing table, see the Cable Guy article titled "Understanding the IPv6 Routing Table" at http://technet.microsoft.com/en-us/library/bb878115.aspx.

An autoconfigured IPv6 address can be in one or more of the states shown in Table-6.

Table-6 Possible States for an Autoconfigured IPv6 Address

State	Description
Tentative	The uniqueness of the address is still being verified using duplicate address detection.
Valid	The address is unique and can now send and receive unicast IPv6 traffic until the Valid Lifetime expires.
Preferred	The address can be used for unicast traffic until the Preferred Lifetime expires.
Deprecated	The address can still be used for unicast traffic during existing communication sessions, but its use is discouraged for new communication sessions.
Invalid	The Valid Lifetime for the address has expired and it can no longer be used for unicast traffic.

Note The Valid and Preferred Lifetime for stateless autoconfigured IPv6 addresses is included in the Router Solicitation message.

For detailed descriptions of how address autoconfiguration, address resolution, router discovery, redirect, duplicate address detection, and neighbor unreachability detection processes are performed, see the white paper, "Introduction to IP Version 6," at http://www.microsoft.com/downloads/details.aspx?FamilyID=CBC0B8A3-B6A4-4952-BBE6-D976624C257C&displaylang=en.

Note To display the state for each autoconfigured IPv6 address on a Windows 7 computer, open a command prompt and type netsh interface ipv6 show addresses at a command prompt.