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IPv6 address space

Updated: January 21, 2005

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

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, which is four times larger than an address in IPv4. 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 (3.4 × 1038) possible addresses.

In the late 1970s when the IPv4 address space was designed, it was unimaginable that it could 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 be consumed. To help put this number in perspective, a 128-bit address space provides 655,570,793,348,866,943,898,599 (6.5 × 1023) addresses for every square meter of the Earth's surface.

It is important to note that the decision to make the IPv6 address 128 bits in length was not so that every square meter of the Earth could have 6.5 x 1023 addresses. Rather, the relatively large size of the IPv6 address is designed to be subdivided into hierarchical routing domains that reflect the topology of the modern-day Internet. The use of 128 bits provides multiple levels of hierarchy and flexibility in designing hierarchical addressing and routing that is currently lacking on the IPv4-based Internet.

The architecture of IPv6 addressing is described in RFC 2373, "IP Version 6 Addressing Architecture."

Current allocation

Similar to the way in which the IPv4 address space is divided, the IPv6 address space is divided based on the value of high order bits in the address. The high order bits and their fixed values are known as a Format Prefix (FP).

The following table shows the allocation of the IPv6 address space by FPs.

 

Allocation Format Prefix (FP) Fraction of the address space

Reserved

0000 0000

1/256

Reserved for NSAP allocation

0000 001

1/128

Aggregatable global unicast addresses

001

1/8

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 remainder of the IPv6 address space is unassigned.

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 represent only 15 percent of the entire IPv6 address space. For more information, see Unicast IPv6 addresses.

Note

  • IPv6 is a rapidly evolving standard. The RFCs referenced might have been made obsolete by newer RFCs.

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