Section 3.8. Link- and Site-Local Addresses

3.8. Link- and Site-Local Addresses

With IPv4, organizations often use IP addresses from the private range as defined in RFC 1918. The addresses reserved for private use should never be forwarded over Internet routers but should instead be confined to the organization's network. For connection to the Internet, Network Address Translation (NAT) maps internal private addresses to publicly registered IPv4 addresses.

The original IPv6 specification allocated two separate address spaces (scopes) for link- and site-local use, both identified by their prefixes. In the meantime, the site-local address has been deprecated. Too many problems arose in the application of this address. A link-local address is for use on a single link and should never be routed. It doesn't need a global prefix and can be used for autoconfiguration mechanisms, for neighbor discovery, and on networks with no routers, so it is useful for creating temporary networks. Let's say you meet your friend in a conference room and you want to share files on your computers. You can connect your computers using a wireless network or a cross-cable between your Ethernet interfaces, and you can share files without any special configuration by using the link-local address.

The replacement for site-local addresses is called unique local IPv6 unicast address , or local IPv6 address for short. It is specified in RFC 4193. These addresses are globally unique but should not be routed to the global Internet. They are designed to be used within corporate sites or confined sets of networks.

The characteristics of unique local IPv6 unicast adresses are the following:

• Have a unique, global prefix, which allows for filtering at network boundaries

• Allow for private connection of networks without the risk of address conflicts and the consequence of having to renumber one of the sites

• Are independent of ISP

• Can be used for internal communication without an Internet connection

• If accidentally routed to the global Internet, no address conflicts arise

• Can be used by applications just like regular Global unicast addresses

The format of these addresses is shown in Figure 3-6.

Figure 3-6. Address formats for link- and site-local use

In hexadecimal notation, a link-local address is identified by the prefix FE80. For the local IPv6 address, RFC 4193 specifies a prefix of FC00::/7. The eights bit is currently set to 1 and specifies local administration of the prefix. Setting the eighth bit to 0 may be used in the future for centrally administrated addresses. For the moment, it was decided to standardize only a locally assigned version. The centrally assigned form may be defined in the future if a strong need is identified.

So for locally administered addresses, we currently have a hexadecimal prefix of FD00::/8. It is followed by the 40 bits for the global ID, which is randomly created to ensure a high probability of uniqueness; 16 bits used for subnet IDs; and 64 bits for the interface identifier. You may still find the site-local address with the prefix FEC0 if you use older implementations, but it should not be used for new implementations anymore.

As mentioned previously, these local addresses should not be routed to the Internet. Border routers should be configured to filter these prefixes. Local addresses should not appear in global DNS servers. They can be used on your internal, private DNS server.

Link-local addresses (FE80) are by default assigned through autoconfiguration. Local IPv6 addresses have to be configured by configuring the local prefix on your routers (Router Advertisement) or through DHCPv6.

If you are interested in the reasons for deprecating the site-local address, refer to RFC 3879.