Local-Use Addresses

A local-use address is a unicast address that has only local routability scope (within the subnet or within a subscriber network), and can have a local or global uniqueness scope. These addresses are intended for use inside of a site for plug and play local communication and for bootstrapping up to the use of global addresses.

Two types of local-use unicast addresses are defined. These are link-local and site-local. The Link-Local-Use is for use on a single link and the Site-Local-Use is for use on a single site. The following table shows the Link-Local-Use address format.

Table 14-3 Link-Local-Use Addresses Format

10 bits	n bits	118-n 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 14-4 Site-Local-Use Addresses

10 bits	n bits	m bits	118-(n+m) 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 the domain in which it is being used. In most cases these will use a node's IEEE-802 48 bit address. The Subnet ID identifies a specific subnet in a site. The combination of the Subnet ID and the interface ID to form a local-use address allows a large private internet to be constructed without any other address allocation.

Local-use addresses allow organizations that are not yet connected to the global Internet to operate without the need to request an address prefix from the global Internet address space. If the organization later connects to the global Internet, it can use its Subnet ID and Interface ID in combination with a global prefix (for example, Registry ID + Provider ID + Subscriber ID) to create a global address. This is a significant improvement over IPv4, which requires sites that use private (non-global) IPv4 addresses to manually renumber when they connect to the Internet. IPv6 automatically does the renumbering.