Unicast IPv6 Addresses

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, also known as global addresses, are identified by the FP of 001. IPv6 global addresses are equivalent to public IPv4 addresses. They are globally routable and reachable on the IPv6 portion of the Internet.

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 is 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 of a global address is the entire IPv6 Internet.

Figure 3-1 shows the structure of an aggregatable global unicast address.

Figure 3-1. The structure of an aggregatable global unicast address

The fields in the aggregatable global unicast address are:

TLA ID — Top-Level Aggregation Identifier. 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, long-haul 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 — Bits that are reserved for future use in expanding the size of either the TLA ID or the NLA ID (defined next). The size of this field is 8 bits.

NLA ID — Next-Level Aggregation Identifier. The size of this field is 24 bits. The NLA ID allows an ISP to create multiple levels of addressing hierarchy within its network to both organize addressing and routing for downstream ISPs and identify organization sites. The structure of the ISP's network is not visible to the default-free routers. The combination of the 001 FP, the TLA ID, the Res field, and the NLA ID form a 48-bit prefix that is assigned to an organization's site that is connecting to the IPv6 portion of the Internet. A site is an organization network or portion of an organization's network that has a defined geographical location (such as an office, an office complex, or a campus).

SLA ID — Site-Level Aggregation Identifier. 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 create 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 organization's network is not visible to the ISP.

Interface ID — Indicates the interface on a specific subnet. The size of this field is 64 bits. The interface ID in IPv6 is equivalent to the node ID or host ID in IPv4.

Billions of Sites

Another way to gauge the practical size of the IPv6 address space is to examine the number of sites that can connect to the IPv6 Internet. With the current FP of 001 and the current definition of the TLA ID (13 bits long) and NLA ID (24 bits long), it is possible to define 237 or 137,438,953,472 possible 48-bit prefixes to assign to sites connected to the Internet. This large number of sites is possible even when we are using only 1/8th of the entire IPv6 address space.

By comparison, using the Internet address classes originally defined for IPv4, it was possible to assign 2,113,389 network IDs to organizations connected to the Internet. The number 2,113,389 is derived from adding up all the possible Class A, Class B, and Class C network IDs and then subtracting the network IDs used for the private address space. Even with the adoption of CIDR to make more efficient use of unassigned Class A and Class B network IDs, the number of possible sites connected to the Internet is not substantially increased nor does it approach the number of possible sites that can be connected to the IPv6 Internet.

Topologies Within Global Addresses

The fields within the global address create a three-level topological structure, as shown in Figure 3-2.

Figure 3-2. The topological structure of the global 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 specifies a unique interface on a subnet within an organization's site.

Local-Use Unicast Addresses

There are two types of local-use unicast addresses:

1. Link-local addresses are used between on-link neighbors and for Neighbor Discovery processes.
2. Site-local addresses are used between nodes communicating with other nodes in the same organization.

Link-Local Addresses

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 autoconfigured on Microsoft Windows .NET Server 2003 family, Windows XP, Windows 2000, Windows Millennium Edition, and Windows 98 computers using the 169.254.0.0/16 prefix. The scope of a link-local address is the local link.

Figure 3-3 shows the structure of the link-local address.

Figure 3-3. The structure of the link-local address

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 Chapter 8, "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.

Site-Local Addresses

Site-local addresses, identified by the FP of 1111 1110 11, are equivalent to the IPv4 private address space (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16). For example, private intranets that do not have a direct, routed connection to the IPv6 Internet can use site-local addresses without conflicting with global addresses. Site-local addresses are not reachable from other sites, and routers must not forward site-local traffic outside the site. Site-local addresses can be used in addition to global addresses. The scope of a site-local address is the site.

Figure 3-4 shows the structure of the site-local address.

Figure 3-4. The structure of the site-local address

Unlike link-local addresses, site-local addresses are not automatically configured and must be assigned either through stateless or stateful address autoconfiguration. For more information, see Chapter 8, "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 divide 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 global address and site-local address share the same structure beyond the first 48 bits of the address. In global addresses, the SLA ID field identifies the subnet within an organization. For site-local addresses, the Subnet ID field performs the same function. Because of this, you can create a subnetted routing infrastructure that is used for both site-local and global addresses.

For example, a specific subnet of your organization can be assigned the global prefix 3FFE:FFFF:4D1C:221A::/64 and the site-local prefix FEC0:0:0: 221A::/64 where the subnet is effectively identified by the SLA ID/Subnet ID value of 221A. While the subnet identifier is the same for both prefixes, routes for both prefixes must still be propagated throughout the routing infrastructure so that addresses based on both prefixes are reachable.

Special IPv6 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 0.0.0.0. The unspecified address is typically used as a source address when a unique address has not yet been determined. 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 127.0.0.1. Packets addressed to the loopback address must never be sent on a link or forwarded by an IPv6 router.

Compatibility Addresses

To aid in the migration from IPv4 to IPv6 and 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 IPv6/IPv4 nodes that are communicating with IPv6 over an IPv4 infrastructure that uses public IPv4 addresses, such as the Internet.

• 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. Windows .NET Server 2003 family and Windows XP IPv6 do not support the use of IPv4-mapped addresses.

• 6over4 address

  An address of the type [64-bit prefix]:0:0:WWXX:YYZZ, where WWXX: YYZZ is the colon hexadecimal representation of w.x.y.z (a public or private IPv4 address), is used to represent a host for the tunneling mechanism known as 6over4.

• 6to4 address

  An address of the type 2002:WWXX:YYZZ:[SLA ID]:[Interface ID], where WWXX:YYZZ is the colon hexadecimal representation of w.x.y.z (a public IPv4 address), is used to represent a node for the tunneling mechanism known as 6to4.

• ISATAP address

  An address of the type [64-bit prefix]:0:5EFE:w.x.y.z, where w.x.y.z is a public or private IPv4 address, is used to represent a node for the address assignment mechanism known as Intra-Site Automatic Tunnel Addressing Protocol (ISATAP).

For more information about IPv6 compatibility addresses, see Chapter 11, "Coexistence and Migration."

NSAP Addresses

To provide a way of mapping Open Systems Interconnect (OSI) NSAP addresses to IPv6 addresses, NSAP addresses use the FP of 0000001 and map the last 121 bits of the NSAP address to an IPv6 address. For more information about the four types of NSAP address mappings, see RFC 1888. Figure 3-5 shows the structure of NSAP addresses for IPv6.

Figure 3-5. The structure of NSAP addresses for IPv6