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Understanding IPv6 (part 3) - Understanding Address Autoconfiguration, Understanding Name Resolution

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5. Understanding Address Autoconfiguration

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

  • Manually using static address assignment

  • Automatically using DHCP, if a DHCP server is present on the subnet (or a DHCP relay agent configured on the subnet)

  • Automatically using Automatic Private IP Addressing (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 DHCPv6 to obtain address information and other configuration settings from a DHCPv6 server.

Note

The DHCP Server service of Windows Server 2003 does not support DHCPv6. Windows Server Code Name “Longhorn” will include support for the DHCPv6 Server role.


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. 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.


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
StateDescription
TentativeThe uniqueness of the address is still being verified using duplicate address detection.
ValidThe address is unique and can now send and receive unicast IPv6 traffic until the Valid Lifetime expires.
PreferredThe address can be used for unicast traffic until the Preferred Lifetime expires.
DeprecatedThe address can still be used for unicast traffic during existing communication sessions, but its use is discouraged for new communication sessions.
InvalidThe 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 in Windows Vista is included in the Router Solicitation message.



Note

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


6. Understanding Name Resolution

The Domain Name System (DNS) is fundamental to how name resolution works on both IPv4 and IPv6 networks. On an IPv4 network, host (A) records are used by name servers (DNS servers) to resolve fully qualified domain names (FQDNs) like server1.contoso.com into their associated IP addresses in response to name lookups (name queries) from DNS clients. In addition, reverse lookups—in which IP addresses are resolved into FQDNs—are supported by using pointer (PTR) records in the in-addr.arpa domain.

Name resolution works fundamentally the same way with IPv6, with the following differences:

  • Host records for IPv6 hosts are AAAA (“quad-A”) records, not A records.

  • The domain used for reverse lookups of IPv6 addresses is ip6.arpa, not in-addr.arpa.

Understanding Name Queries

Because Windows Vista’s dual-layer TCP/IP stack means that both IPv4 and IPv6 are enabled by default, DNS name lookups by Windows Vista client computers can involve the use of both A and AAAA records. (This is true only if your name servers support IPv6, which is the case with the DNS Server role for Windows Server 2003.) By default, the DNS client component on Windows Vista uses the following procedure when performing a name lookup using a particular interface:

  1. The client computer checks to see if it has a non-link-local IPv6 address assigned to the interface. If it has no non-link-local addresses assigned, the client sends a single name lookup to the name server to query for A records and does not query for AAAA records. If the only non-link-local address assigned to the interface is a Teredo address, the client again does not query for AAAA records. (The Teredo client in Windows Vista has been explicitly built not to automatically perform AAAA lookups or register with DNS to prevent overloading of DNS servers.)

  2. If the client computer has a non-link-local address assigned to the interface, the client sends a name lookup to query for A records.

    1. If the client then receives a response to its query (not an error message), it follows with a second lookup to query for AAAA records.

    2. If the client receives no response or receives any error message (except for Name Not Found), it does not send a second lookup to query for AAAA records.

Understanding Name Registration

DNS servers running Windows Server 2003 can dynamically register both A and AAAA records for Windows Vista client computers. Dynamic registration of DNS records simplifies the job of maintaining name resolution on networks running the Active Directory directory service. When a Windows Vista client computer starts up on a network, the DNS Client service tries to register the following records for the client:

  • A records for all IPv4 addresses assigned to all interfaces configured with the address of a DNS server

  • AAAA records for all IPv6 addresses assigned to all interfaces configured with the address of a DNS server

  • PTR records for all IPv4 addresses assigned to all interfaces configured with the address of a DNS server

Note

AAAA records are not registered for link-local IPv6 addresses that have been assigned to interfaces using address autoconfiguration.


PTR Records and IPv6

Windows Vista client computers do not try to register PTR records for IPv6 addresses assigned to interfaces on the computer. If you want to enable clients to perform reverse lookups for Windows Vista computers using IPv6, you must manually create a reverse lookup zone for the ip6.arpa domain on your DNS servers and then manually add PTR records to this zone. For detailed steps on how to do this, see “IPv6 for Microsoft Windows: Frequently Asked Questions,” found at http://www.microsoft.com/technet/itsolutions/network/ipv6/ipv6faq.mspx.

However, PTR records for reverse lookups using IPv6 are not often used, because the namespace for reverse queries is formed by using each hexadecimal digit in the colon-hexadecimal representation of an IPv6 address as a separate level in the reverse domain hierarchy. For example, the PTR record associated with the IPv6 address 2001DB8::D3:00FF:FE28:9C5A, whose full representation is 2001:0DB8:0000:0000:00D3:00FF:FE28:9C5A, would be expressed as A.5.C.9.8.2.E.F.F.F.0.0.3.D.0.0.0.0.0.0.0.0.0.0.8.B.D.0.1.0.0.2.IP6.ARPA. The performance cost of resolving such a representation would generally be too high for most DNS server implementations.


By default, DNS servers running Windows Server 2003 do not listen for DNS traffic sent over IPv6. To enable these DNS servers to listen for IPv6 name registrations and name lookups, you must first configure the servers using the dnscmd /config /EnableIPv6 1 command. You must then manually configure each Windows Vista client computer with the unicast IPv6 addresses of your DNS servers using the netsh interface ipv6 add dns interface=NameOrIndex address=IPv6Address index=PreferenceLevel command. (DHCP servers running Windows Server 2003 currently do not support stateful address assignment using DHCPv6.)

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