This is the most important address type because unicast addresses are the “normal” addresses identifying the common computers, printers and other devices connected to the network.

Let’s look at the most important subtype of unicast addresses first - at the global unicast addresses defined by RFC 3587 [RFC3587]. The internal unicast address structure defined by this RFC is quite simple. It contains just three parts as depicted in Figure 3-1: global routing prefix, subnet ID, and interface ID.

Global routing prefix

is the network address in IPv4 parlance. This address prefix identifies uniquely the network connected to the Internet.

Subnet ID

is the identifier of a subnet. The end-network may be partitioned into to subnets (for example every building of some institution may hold separate subnet). This part of the address serves to identify individual subnets.

Interface ID

holds the identifier of single network interface. Interface identifiers are unique inside the same subnet only, there may be devices holding the same interface ID in different subnets. Internet standards request the modified EUI-64 (described below) to play the role of interface ID.

In reality the address structure is even more simple, because all used addressing schemes have the common length of the global prefix (48 bits) and subnet identifier (16 bits). In consequence the typical unicast address has the structure showed in Figure 3-2.

Not all unicast addresses are global. Some of them are limited just to a single physical (layer 2) network. These link-local addresses are distinguished by prefix fe80::/10. They can be used for intralink communication only – both the sender and recipient of the datagram must be connected to the same local network. A router must not forward any datagram having such a destination address.

Theses addresses are used in some mechanisms, such as the autoconfiguration of network parameters.RFC 3513 actually defines two scoping levels: link-local (prefix fe80::/10) and site-local (fec0::/10) addresses. But due to a long-term lack of consensus on the definition of “site” RFC 3879 [RFC3879]deprecated the usage of site-local addresses and prohibits new IPv6 implementations to handle the fec0::/10 prefix. RFC 3879 states that the given prefix is reserved for potential future usage.

Note:

Although, in theory, site-local addresses have been deprecated by RFC 3879, many IPv6 implementations and IPv6 applications still use site-local prefixes and this will probably remain true for some time. Indeed, some of the configuration examples in this book contain site-local addresses.

Consequently, unicast addresses have just two scope levels: link-local (starting with fe80::/10) or global (all the others).