IPv6 Addresses
Local Addresses

Local addresses are similar to private IPv4 addresses and are used on a single link. These addresses are not forwarded in the global Internet and registered or centrally coordinated. They are used in combination with NAT today. The local addresses include:

Local addresses have the following format:

Local Addresses

Link-Local Unicast Addresses

Link-Local addresses are designed to be used for addressing on a single link for purposes such as automatic address configuration, neighbor discovery, or when no routers are present. Routers must not forward any datagrams with Link-Local source or destination addresses to other links. RFC 1918 defines the Link-Local Unicast addresses.

Example of Link-Local Address

Unique-Local Unicast Addresses

Unique-Local addresses are designed to be used for the case when there are more networks (subnets) that the administrator considers as a single network and where he also wants to use both Link-Local addresses and these addresses. These subnets are usually connected using the core networks and the use of Unique-Local addresses does not cause the problems when the data are transferred over these networks. It is because each subnet has its own prefix and thus different local addresses. The probability that the two subnets choose the same Unique-Local addresses is approximately 10-12.

Example of Unique-Local Address

RFC 4193 defines the Unique-Local Unicast addresses. These addresses start with the prefix FC::/7. The L flag that is followed the prefix identifies whether the address is generated locally or otherwise. Currently, the Unique-Local addresses are generated only locally. It means that the L flag is set to 1 and therefore the all Unique-Local addresses start with the prefix FD::/8 today.

The next 40 bits contain a global identifier that is generated randomly. RFC 4193 recommends generating this identifier from the current time, node address and SHA-1 algorithm. The prefix FC::/7 together with a global identifier create a prefix with a length of 48 bits. 16-bit Subnet ID and 64-bit Interface ID that is generated according to the modified EUI-64 standard follow the 48-bit prefix.