A key new feature of IPv6 is autoconfiguration [RFC2462], where nodes configure themselves without prior static configuration as soon as they are connected on a network. Their first task is to find their address by concatenating the prefix of the network with the host part of the address.
IPv6 uses the uniqueness of link-layer addresses by inserting the link-layer address of the interface inside the host part of the address. The layer 2 address (e.g. Ethernet MAC address) is effectively embedded in the layer 3 (e.g. IP) address. This mechanism, called interface identifier, should avoid simultaneous use of the same address chosen by two nodes on the same link.
Figure 5.1 shows the process of composing an interface identifier. The IEEE EUI-64 [IEEEEUI64] procedure is used with a slight change.