Currently, Internet Assigned Numbers Authority (IANA) and The Internet Committee for Assigned Names and Numbers (ICANN) allocates IPv6 address blocks to the five RIRs. Only global unicast addresses with the first three bits of 001 or 2000::/3 are being assigned to various Internet address registries. This is only very small portion of available IPv6 addresses. A global unicast address has three parts which are illustrated in the figure below:-
- Global routing prefix
- Subnet ID
- Interface ID
Global Routing Prefix
This is the network portion of the global IPv6 address, which is assigned by the provider, such as an ISP, to a customer or site. In general, RIRs assign a /48 global routing prefix to customers as shown in the above figure. This can use everyone from business networks to individual households.
The figure shows the structure of a global unicast address using a /48 global routing prefix with 16-bit subnet ID. /48 prefixes are the most common global routing prefixes assigned. The range of global IPv6 prefix in the first hextet is 0010 0000 0000 (2000) to 0011 1111 1111 (3FFF).
The Subnet ID is used by departments to recognize subnets within its site. The larger the subnet ID, the more subnets available.
This is the host portion of the IPv6 address like the host portion of an IPv4 address. The term Interface ID is used for the reason that a single host may have multiple interfaces, each having one or more IPv6 addresses. It is extremely recommended that in most cases /64 subnets should be used.
Example of IPv6 Address
shown in the above IP address, in a /64 global unicast address the first four hextets are for the network portion of the address which is indicated in red, with the fourth hextets indicating the Subnet ID. The remaining four hextets are for the Interface ID.