Comparing Address Types

Applies To: Windows Server 2003, Windows Server 2003 with SP1

If you install IPv6 on a computer that is not connected to a network in which an IPv6 router is present, the computer automatically configures a link-local IPv6 address, which is a type of address that allows you to communicate with computers on your subnet. If you connect your computer to a subnet in which an IPv6 router is present, the router assigns your computer an IPv6 global or site-local address. The site-local address allows your computer to communicate within your intranet. The global address allows your computer to communicate with computers on the IPv6 Internet.

The left-most bits of an IP address are called the format prefix (FP), which indicates the specific type of IPv6 address. IPv6 accommodates many address types, including the following:

• Unicast addresses. Provide point-to-point, directed communication between two hosts on a network.

• Multicast addresses. Provide a method for sending a single IP packet to multiple hosts in a group. A multicast address is used for one-to-many communication.

• Anycast addresses. Provide a method of delivering a packet to the nearest member of a group. Currently, anycast addresses are used only as destination addresses and are assigned only to routers. An anycast address is used for one-to-one-of-many communication.

Table G.3 compares some basic elements of IPv4 and IPv6 addressing.

Table G.3 Comparison of IPv4 and IPv6 Addressing Elements

1 An A resource record, which is stored on your DNS servers, enables mapping from a host name to an IPv4 32-bit address.

2 AAAA (quad-A) resource records enable mapping from a host name to an IPv6 128-bit address.

Unicast Addresses

Unicast addresses identify a single interface within the scope of a particular type of unicast address. The scope of an address is the region of the IPv6 network over which the address is unique. With the appropriate unicast routing topology, packets addressed to a unicast address are delivered only to a single interface.

The following are types of unicast IPv6 addresses:

• Aggregatable global unicast addresses. Identified by the format prefix (FP) of 001, these addresses are equivalent to public IPv4 addresses.

• Local-use unicast addresses. Provide two types of addresses:

  • Link-local addresses. Identified by the FP of 1111 1110 10, these addresses are used by nodes when they are communicating with neighboring nodes on the same link.

  • Site-local addresses. Identified by the FP of 1111 1110 11, these addresses are equivalent to the IPv4 private address space. Use these addresses between nodes that communicate with other nodes in the same site.

• Unspecified address. Used only to indicate the absence of an address; this type of address cannot be assigned to a node. The IPv6 unspecified address, 0:0:0:0:0:0:0:0 or ::, is equivalent to the IPv4 unspecified address of 0.0.0.0.

• Loopback address. Used to identify a loopback interface, which enables a node to send packets to itself. The IPv6 loopback address, 0:0:0:0:0:0:0:1 or ::1, is equivalent to the IPv4 loopback address of 127.0.0.1.

• Transition, or compatibility, addresses. Provided to help you migrate from IPv4 to IPv6; these addresses allow both types of hosts to coexist on your network.

Types of Transition IPv6 Addresses

To help you transition from IPv4 to IPv6 and to facilitate the coexistence of both types of hosts, IPv6 defines the following transition IPv6addresses.

IPv4-compatible addresses

IPv4-compatible addresses are used by IPv6/IPv4 nodes that communicate with IPv6 over an IPv4 infrastructure. IPv6/IPv4 nodes are nodes that run both the IPv4 and IPv6 protocols. The format for an IPv4-compatible address is 0:0:0:0:0:0:w.x.y.z or ::w.x.y.z (where w.x.y.z is the dotted-decimal representation of a public IPv4 address). The IPv6 protocol for Windows XP and Windows Server 2003 provides support for IPv4-compatible addresses, but support is not enabled by default.

IPv4-mapped addresses

IPv4-mapped addresses are used to represent an IPv4-only node to an IPv6 node. The IPv4-mapped address is never used as a source or destination address for an IPv6 packet. It is used only for internal representation. The format for an IPv4-mapped address is 0:0:0:0:0:FFFF:w.x.y.z or ::FFFF:w.x.y.z. The IPv6 protocol for Windows XP and Windows Server 2003 does not support IPv4-mapped addresses.

6to4 addresses

6to4 addresses are used for communicating between two nodes that are running both IPv4 and IPv6 over the Internet. The 6to4 address is formed by combining the prefix 2002::/16 with the 32 bits of the public IPv4 address of the node or the site of the node, thus forming a 48-bit prefix. For example, for the IPv4 address of 131.107.0.1, the 6to4 address prefix is 2002:836B:1::/48 (where 836B:1 is the colon hexadecimal notation for 131.107.0.1). Support for 6to4 addresses is provided by the IPv6 Helper service (known as the 6to4 service) that is included with the IPv6 protocol for Windows XP and Windows Server 2003.

Global Addresses

Global addresses, which are identified by an FP of 001, and which are also called aggregatable global unicast addresses, are equivalent to public IPv4 addresses. Global addresses are globally routable and reachable on the IPv6 Internet.

As the name implies, you can aggregate, or summarize, global addresses to produce an efficient routing infrastructure. Unlike the current IPv4-based Internet, which has a mixture of both flat and hierarchical routing, the IPv6-based Internet is designed to support efficient hierarchical addressing and routing. The scope of a global address, which is the region of the IPv6 internetwork over which the address is unique, is the entire IPv6 Internet.

Figure G.1 illustrates the structure of an IPv6 global address.

TLA ID field

Indicates the Top Level Aggregation Identifier (TLA ID) for the address. The size of this field is 13 bits. The TLA ID identifies the highest level in the routing hierarchy. TLA IDs are administered by the Internet Assigned Numbers Authority (IANA) and allocated to local Internet registries that, in turn, allocate individual TLA IDs to large, global Internet service providers (ISPs). A 13-bit field allows up to 8,192 different TLA IDs. Routers in the highest level of the IPv6 Internet routing hierarchy (called default-free routers) do not have a default route — only routes with 16-bit prefixes corresponding to the allocated TLA IDs and additional entries for routes based on the TLA ID assigned to the routing region where the router is located.

Res field

Reserves space for future expansion of either the TLA ID or the NLA ID field. The size of this field is 8 bits.

NLA ID field

Indicates the Next Level Aggregation Identifier (NLA ID) for the address. The NLA ID identifies a specific customer site. The size of this field is 24 bits. The NLA ID allows an ISP to create multiple levels of addressing hierarchy to organize addressing and routing and to identify sites. The structure of the ISP's network is not visible to default-free routers.

SLA ID field

Indicates the Site Level Aggregation Identifier (SLA ID) for the address. The SLA ID is used by an individual organization to identify subnets within its site. The size of this field is 16 bits. An organization can use this field to create 65,536 subnets or multiple levels of addressing hierarchy. Being assigned an SLA Id is equivalent to being allocated an IPv4 Class A network ID (assuming that the last octet is used for identifying nodes on subnets). The structure of the customer's network is not visible to the ISP.

Interface ID field

Identifies the interface of a node on a specific subnet. The size of this field is 64 bits.

For more information about IPv6 addressing, see RFC 2373, IP Version 6 Addressing Architecture, at the Internet Engineering Task Force Web site.