The IPv6 Address Space

The most obvious distinguishing feature of IPv6 is its use of much larger addresses. The size of an address in IPv6 is 128 bits, a bit-string that is four times longer than the 32-bit IPv4 address. A 32-bit address space allows for 2³², or 4,294,967,296, possible addresses. A 128-bit address space allows for 2¹²⁸, or 340,282,366,920,938,463,463,374,607,431,768,211,456 (or 3.4 x 10³⁸), possible addresses.

In the late 1970s, when the IPv4 address space was designed, it was unimaginable that it could ever be exhausted. However, due to changes in technology and an allocation practice that did not anticipate the recent explosion of hosts on the Internet, the IPv4 address space was consumed to the point that by 1992, it was clear a replacement would be necessary.

With IPv6, it is even harder to conceive that the IPv6 address space will ever be consumed. To help put this number in perspective, a 128-bit address space provides 665,570,793,348,866,943,898,599 (6.65 x 10²³) addresses for every square meter of the Earth's surface.

It is important to remember that the decision to make the IPv6 address 128 bits in length was not so that every square meter of the Earth could have 6.65 x 10²³ addresses. Rather, the relatively large size of the IPv6 address is designed to be divided into hierarchical routing domains that reflect the topology of the modern-day Internet. The use of 128 bits allows for multiple levels of hierarchy and flexibility in designing hierarchical addressing and routing that is currently lacking on the IPv4-based Internet.

Addresses Per Square Meter of the Earth

The number of 6.65 x 10²³ addresses for every square meter of the Earth's surface is derived from the fact that the surface of the Earth is approximately 197,399,019 square miles and there are 2.59 x 10⁶ square meters per square mile. So, the Earth's surface is 197,399,019 x 2.59 x 10⁶, or 511,263,971,197,990 square meters.

Therefore, there are 340,282,366,920,938,463,463,374,607,431,768, 211,456 / 511,263,971,197,990, or 665,570,793,348,866,943,898,599 (or 6.65 x 10²³) addresses for each square meter of the Earth's surface.

It is easy to get lost in the vastness of the IPv6 address space. As we will discover, the unthinkably large 128-bit IPv6 address that is assigned to an interface on a typical IPv6 host is composed of a 64-bit subnet identifier and a 64-bit interface identifier (a 50-50 split between subnet space and interface space). The 64 bits of subnet identifier leave enough addressing room to satisfy the addressing requirements of three levels of Internet service providers (ISPs) between your organization and the backbone of the Internet and the addressing needs of your organization. The 64 bits of interface identifier accommodate the mapping of current and future link-layer media access control (MAC) addresses.

Current Allocation

Similar to the way in which the IPv4 address space was divided into unicast addresses (using Internet address classes) and multicast addresses, the IPv6 address space is divided on the basis of the value of high-order bits. The high-order bits and their fixed values are known as a Format Prefix (FP).

Table 3-1 lists the allocation of the IPv6 address space by FPs as defined in RFC 2373.

Table 3-1. Current Allocation of the IPv6 Address Space

Allocation	Format Prefix (FP)	Fraction of the Address Space
Reserved	0000 0000	1/256
Unassigned	0000 0001	1/256
Reserved for Network Service Access Point (NSAP) allocation	0000 001	1/128
Unassigned	0000 010	1/128
Unassigned	0000 011	1/128
Unassigned	0000 1	1/32
Unassigned	0001	1/16
Aggregatable global unicast addresses	001	1/8
Unassigned	010	1/8
Unassigned	011	1/8
Unassigned	100	1/8
Unassigned	101	1/8
Unassigned	110	1/8
Unassigned	1110	1/16
Unassigned	1111 0	1/32
Unassigned	1111 10	1/64
Unassigned	1111 110	1/128
Unassigned	1111 1110 0	1/512
Link-local unicast addresses	1111 1110 10	1/1024
Site-local unicast addresses	1111 1110 11	1/1024
Multicast addresses	1111 1111	1/256

The current set of unicast addresses that can be used with IPv6 nodes consists of aggregatable global unicast addresses, link-local unicast addresses, and site-local unicast addresses. These addresses represent only 12.7 percent of the entire IPv6 address space.