The global unicast address is globally unique in the Internet. The example IPv6 address that is shown in Prefixes in IPv6 is a global unicast address. The next figure shows the scope of the global unicast address, as compared to the parts of the IPv6 address.
The site prefix defines the public topology of your network to a router. You obtain the site prefix for your enterprise from an ISP or Regional Internet Registry (RIR).
IN IPv6, the subnet ID defines an administrative subnet of the network and is up to 16 bits in length. You assign a subnet ID as part of IPv6 network configuration. The subnet prefix defines the site topology to a router by specifying the specific link to which the subnet has been assigned.
IPv6 subnets are conceptually the same as IPv4 subnets, in that each subnet is usually associated with a single hardware link. However, IPv6 subnet IDs are expressed in hexadecimal notation, rather than in dotted decimal notation.
The interface ID identifies an interface of a particular node. An interface ID must be unique within the subnet. IPv6 hosts can use the Neighbor Discovery protocol to automatically generate their own interface IDs. Neighbor Discovery automatically generates the interface ID, based on the MAC or EUI-64 address of the host's interface. You can also manually assign interface IDs, which is recommended for IPv6 routers and IPv6-enabled servers. For instructions on how to create a manual EUI-64 address, refer to RFC 3513 Internet Protocol Version 6 (IPv6) Addressing Architecture.