Local-Use Addresses

A local-use address is a unicast address that has only local routability scope. A local-use address can only be used within the subnet or within a subscriber network. These addresses are intended for use inside of a site for plug and play local communication. These addresses are also used for bootstrap operations for the use of global addresses.

The two types of local-use unicast addresses are link-local and site-local. The Link-Local-Use address is for a single data link layer medium, such as an Ethernet or Frame Relay link. The Site-Local-Use is for use on a single site. The following table shows the Link-Local-Use address format.

Table 1–3 Link-Local-Use Addresses Format

10 bits	54 bits	64 bits
1111111010	0	Interface ID

Link-Local-Use addresses are used for addressing on a single link for purposes such as auto-address configuration.

The following table shows the Site-Local-Use address format.

Table 1–4 Site-Local-Use Addresses

10 bits	38 bits	16 bits	64 bits
1111111011	0	Subnet ID	Interface ID

For both types of local-use addresses, the Interface ID is an identifier that must be unique in its domain. In most instances, the identifier uses a node's IEEE-802 48–bit address. The Subnet ID identifies a specific subnet in a site. The Subnet ID and the interface ID form a local-use address. Consequently, a large private internet can be constructed without any other address allocation.

Organizations that are not yet connected to the global Internet can use local-use addresses. Local-use addresses enable organizations to operate without the need to request an address prefix from the global Internet address space. If the organization later connects to the Internet, the Subnet ID, Interface ID, and a global prefix can be used to create a global address. For example, the organization can use the Registry ID, Provider ID, and the Subscriber ID to create a global address. This enhancement is a significant improvement over IPv4. IPv4 requires sites that use private (non-global) IPv4 addresses to manually renumber when sites connect to the Internet. IPv6 automatically does the renumbering.