Understanding Unicast Addresses

Unicast addresses are addresses that identify a single interface. IPv6 has several types of unicast addresses:

• Global unicast address An address that is globally routable over the IPv6-enabled portion of the Internet. Therefore, the scope of a global address is the entire Internet, and global addresses in IPv6 correspond to public (non-RFC 1918) addresses used in IPv4. The address prefix currently used for global addresses as defined in RFC 3587 is 2000::/3, and a global address has the following structure:
  • The first 48 bits of the address are the global routing prefix specifying your organization's site. (The first three bits of this prefix must be 001 in binary notation.) These 48 bits represent the public topology portion of the address, which represents the collection of large and small Internet service providers (ISPs) on the IPv6 Internet and which is controlled by these ISPs through assignment by the Internet Assigned Numbers Authority (IANA).
  • The next 16 bits are the subnet ID. Your organization can use this portion to specify up to 65,536 unique subnets for routing purposes inside your organization's site. These 16 bits represent the site topology portion of the address, which your organization has control over.
  • The final 64 bits are the interface ID and specify a unique interface within each subnet.
• Link-local unicast address An address that can be used by a node for communicating with neighboring nodes on the same link. Therefore, the scope of a link-local address is the local link on the network; link-local addresses are never forwarded beyond the local link by IPv6 routers. Because link-local addresses are assigned to interfaces using IPv6 address autoconfiguration, link-local addresses in IPv6 correspond to Automatic Private IP Addressing (APIPA) addresses used in IPv4 (which are assigned from the address range 169.254.0.0/16). The address prefix used for link-local addresses is FE80::/64, and a link-local address has the following structure:
  • The first 64 bits of the address are always FE80:0:0:0 (which will be shown as FE80::).
  • The last 64 bits are the interface ID and specify a unique interface on the local link.
  Link-local addresses can be reused-in other words, two interfaces on different links can have the same address. This makes link-local addresses ambiguous; an additional identifier called the zone ID (or scope ID) indicates to which link the address is either assigned or destined. In Windows 7, the zone ID for a link-local address corresponds to the interface index for that interface. You can view a list of interface indexes on a computer by typing netsh interface ipv6 show interface at a command prompt. For more information on the zone ID, see the section titled "Displaying IPv6 Address Settings" later in this tutorial.
• Unique local unicast address Because a site-local address prefix can represent multiple sites within an organization, it is ambiguous and not well suited for intraorganizational routing purposes. Therefore, RFC 4193 currently proposes a new type of address called a unique local unicast address. The scope of this address is global to all sites within the organization, and using this address type simplifies the configuration of an organization's internal IPv6 routing infrastructure. A unique local address has the following structure:
  • The first seven bits of the address are always 1111 110 (binary) and the eighth bit is set to 1, indicating a unique local address. This means that the address prefix is always FD00::/8 for this type of address.
  • The next 40 bits represent the global ID, a randomly generated value that identifies a specific site within your organization.
  • The next 16 bits represent the subnet ID and can be used for further subdividing the internal network of your site for routing purposes.
  • The last 64 bits are the interface ID and specify a unique interface within each subnet.

Note Site-local addresses have been deprecated by RFC 3879 and are replaced by unique local addresses.