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Unicast IPv6 Addresses

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A unicast address identifies a single interface within the scope of the unicast address type. The following list shows the types of IPv6 addresses:

  • Aggregatable global unicast addresses
  • Link-local addresses
  • Special addresses, including unspecified and loopback addresses
  • Compatibility addresses, including 6to4 addresses

With the appropriate unicast routing topology, packets addressed to a unicast address are delivered to a single interface.

Aggregatable global unicast addresses, also known as global addresses, are identified by the Format Prefix of 001. Addresses of this type are designed to be aggregated or summarized to produce an efficient routing infrastructure. They are equivalent to public IPv4 addresses. 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.

Aggregatable global unicast addresses are globally routable and reachable on the IPv6 portion of the Internet. The region of the Internet over which the aggregatable global unicast address is unique (the scope) is the entire IPv6 Internet.

The following illustration shows the fields of the aggregatable global unicast address.


The following table shows the fields in the aggregatable global unicast.

Field Description

Format Prefix

Aggregatable global unicast addresses are identified by the Format Prefix of 001.


Indicates the Top Level Aggregator (TLA) for the address. The TLA identifies the highest level in the routing hierarchy. TLAs are administered by 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 do not have a default route. These default-free routers have routes with 16-bit prefixes that correspond to the allocated TLAs.


Reserved for future use in expanding the size of either the TLA ID or the Next Level Aggregator (NLA) ID.


Indicates the Next Level Aggregator (NLA) for the address, and is used to identify a specific customer site. 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.


Indicates the Site Level Aggregator (SLA) for the address, and is used by an organization to identify subnets within its site. The organization can use the 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

Indicates the interface of a node on a specific subnet.

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


The following table shows the topological Internet routing structure and the associated definitions.

Topology Definition


The collection of larger and smaller ISPs that provide access to the IPv6 Internet.


The collection of subnets within an organization's site.

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.

Link-local addresses 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. An IPv6 router never forwards link-local traffic beyond the 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, see IPv6 Address Autoconfiguration.

Link-local addresses are identified by the Format Prefix of 1111 1110 10. The address always begins with FE80. With the 64-bit interface identifier, the prefix for link-local addresses is always FE80::/64.

For information about how to use link-local addresses, see Single Subnet with Link-Local Addresses.

The following table shows the special IPv6 addresses.

Special Address Description

Unspecified address

The unspecified address, 0:0:0:0:0:0:0:0 or ::, indicates the absence of an address, and is typically used as a source address for packets that are attempting to verify the uniqueness of a tentative address. It is equivalent to the IPv4 unspecified address of 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, identifies 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.

IPv6 provides 6to4 addresses to aid in the migration from IPv4 to IPv6 and to facilitate the coexistence of both types of hosts. The 6to4 address is used for communicating between two nodes running both IPv4 and IPv6 over an IPv4 routing infrastructure. 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).


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