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Unicast IPv6 addresses

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Unicast IPv6 addresses

A unicast address identifies a single interface within the scope of the type of unicast address. With the appropriate unicast routing topology, packets addressed to a unicast address are delivered to a single interface. The following types of addresses are unicast IPv6 addresses:

  • Aggregatable global unicast addresses

  • Link-local addresses

  • Site-local addresses

  • Special addresses

  • Compatibility addresses

  • NSAP addresses

Aggregatable global unicast addresses

Aggregatable global unicast addresses, identified by the Format Prefix (FP) of 001, are equivalent to public IPv4 addresses. They are globally routable and reachable on the IPv6 Internet. Aggregatable global unicast addresses are also known as global addresses.

As the name implies, aggregatable global unicast addresses are designed to be aggregated or summarized 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 has been designed from its foundation to support efficient, hierarchical addressing and routing. The scope, the region of the IPv6 internetwork over which the address is unique, of an aggregatable global unicast address is the entire IPv6 Internet.

Structure of aggregatable global unicast address

The fields in the aggregatable global unicast address are described as follows:

  • TLA ID

    The TLA ID field indicates the Top Level Aggregation Identifier (TLA ID) for the address. The size of this field is 13 bits. The TLA identifies the highest level in the routing hierarchy. TLAs are administered by 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 that correspond to the allocated TLAs.

  • Res

    The Res field is reserved for future use in expanding the size of either the TLA ID or the NLA ID. The size of this field is 8 bits.

  • NLA ID

    The NLA ID field indicates the Next Level Aggregation Identifier (NLA ) for the address. The NLA ID is used to identify 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 the default-free routers.

  • SLA ID

    The 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. The organization can use these 16 bits within its site to create 65,536 subnets or multiple levels of addressing hierarchy and an efficient routing infrastructure. With 16 bits of subnetting flexibility, an aggregatable global unicast prefix assigned to an organization is equivalent to that organization 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

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

The following illustration shows how the fields within the aggregatable global unicast address create a three-level topological structure.

Structure of aggregatable global unicast address

The public topology is the collection of larger and smaller ISPs that provide access to the IPv6 Internet. The site topology is the collection of subnets within an organization's site. The interface identifier identifies a specific interface on a subnet within an organization's site. For more information about aggregatable global unicast addresses, see RFC 2374, "An IPv6 Aggregatable Global Unicast Address Format."

Local-use unicast addresses

There are two types of local-use unicast addresses:

  1. Link-local addresses, which are used between on-link neighbors and for Neighbor Discovery processes.

  2. Site-local addresses, which are used between nodes that communicate with other nodes in the same site.

Link-local addresses, identified by the FP of 1111 1110 10, are used by nodes when communicating with neighboring nodes on the same link. For example, on a single link IPv6 network with no router, link-local addresses are used to communicate between hosts on the link. Link-local addresses are equivalent to Automatic Private IP Addressing (APIPA) IPv4 addresses (using the prefix). The scope of a link-local address is the local link. A link-local address is required for Neighbor Discovery processes and is always automatically configured, even in the absence of all other unicast addresses. For more information about the address autoconfiguration process for link-local addresses, see IPv6 address autoconfiguration.

Link-local addresses always begin with FE80. With the 64-bit interface identifier, the prefix for link-local addresses is always FE80::/64. An IPv6 router never forwards link-local traffic beyond the link.

For information about how to use link-local addresses, see Single subnet with link-local addresses.

Site-local addresses

Site-local addresses, identified by the FP of 1111 1110 11, are equivalent to the IPv4 private address space (,, and For example, private intranets that do not have a direct, routed connection to the IPv6 Internet can use site-local addresses without conflicting with aggregatable global unicast addresses. Site-local addresses are not reachable from other sites, and routers must not forward site-local traffic outside of the site. Site-local addresses can be used in addition to aggregatable global unicast addresses. The scope of a site-local address is the site (the organization internetwork).

Unlike link-local addresses, site-local addresses are not automatically configured and must be assigned either through stateless or stateful address configuration processes. For more information, see IPv6 address autoconfiguration.

The first 48-bits are always fixed for site-local addresses, beginning with FEC0::/48. After the 48 fixed bits is a 16-bit subnet identifier (Subnet ID field) that provides 16 bits with which you can create subnets within your organization. With 16 bits, you can have up to 65,536 subnets in a flat subnet structure, or you can subdivide the high-order bits of the Subnet ID field to create a hierarchical and aggregatable routing infrastructure. After the Subnet ID field is a 64-bit Interface ID field that identifies a specific interface on a subnet.

The aggregatable global unicast address and site-local address share the same structure beyond the first 48 bits of the address. In aggregatable global unicast addresses, the SLA ID identifies the subnet within an organization. For site-local addresses, the Subnet ID performs the same function. Because of this, you can assign a specific subnet number to identify a subnet that is used for both site-local and aggregatable global unicast addresses.

Special addresses

The following are special IPv6 addresses:

  • Unspecified address

    The unspecified address (0:0:0:0:0:0:0:0 or ::) is used only to indicate the absence of an address. It is equivalent to the IPv4 unspecified address of The unspecified address is typically used as a source address for packets that are attempting to verify the uniqueness of a tentative address. The unspecified address is never assigned to an interface or used as a destination address.

  • Loopback address

    The loopback address (0:0:0:0:0:0:0:1 or ::1) is used to identify a loopback interface, enabling a node to send packets to itself. It is equivalent to the IPv4 loopback address of Packets addressed to the loopback address are never sent on a link or forwarded by an IPv6 router.

Compatibility addresses

To aid in the migration from IPv4 to IPv6 and facilitate the coexistence of both types of hosts, the following addresses are defined:

  • IPv4-compatible address

    The IPv4-compatible address, 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), is used by dual-stack nodes that are communicating with IPv6 over an IPv4 infrastructure. Dual-stack nodes are nodes with both IPv4 and IPv6 protocols. When the IPv4-compatible address is used as an IPv6 destination, IPv6 traffic is automatically encapsulated with an IPv4 header and sent to the destination by using the IPv4 infrastructure.

  • IPv4-mapped address

    The IPv4-mapped address, 0:0:0:0:0:FFFF:w.x.y.z or ::FFFF:w.x.y.z, is used to represent an IPv4-only node to an IPv6 node. It is used only for internal representation. The IPv4-mapped address is never used as a source or destination address for an IPv6 packet. The IPv6 protocol does not support the use of IPv4-mapped addresses.

  • 6to4 address

    The 6to4 address is used for communicating between two nodes 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, forming a 48-bit prefix. For example, for the IPv4 address of, the 6to4 address prefix is 2002:836B:1::/48. For more information about 6to4, see IPv6 traffic between nodes in different sites across the Internet (6to4).

NSAP addresses

To provide a means for mapping Network Service Access Point (NSAP) addresses to IPv6 addresses, NSAP addresses use the FP of 0000001 and map the last 121 bits of the IPv6 address to an NSAP address. For more information about the four types of NSAP address mappings, see RFC 1888, "OSI NSAPs and IPv6."


  • IPv6 is a rapidly evolving standard. The RFCs referenced might have been made obsolete by newer RFCs.