# IPv6 address
An IPv6 address is a 128-bit identifier assigned to an interface of a node running version 6 of the [Internet Protocol](https://www.edgepedia.org/internet-protocol), replacing the 32-bit addresses of IPv4 and providing roughly \( 3.4 \times 10^{38} \) possible addresses, which the [Internet Society](https://www.edgepedia.org/internet-society) describes as 340 trillion trillion trillion.[1](https://www.rfc-editor.org/info/rfc2460/) • [2](https://www.internetsociety.org/deploy360/ipv6/faq/) The format and semantics of these addresses are specified in RFC 4291, the IP Version 6 Addressing Architecture, published in February 2006 by Robert Hinden and Steve Deering; it obsoleted RFC 3513 and has itself been updated by six later RFCs covering text representation, translation, interface identifiers, and multicast scopes.[3](https://www.rfc-editor.org/info/rfc4291/)
| Key fact | Detail |
|---|---|
| Size | 128 bits, expanded from IPv4's 32 bits to support more addressing hierarchy, more addressable nodes, and simpler autoconfiguration[1](https://www.rfc-editor.org/info/rfc2460/) |
| Defining standard | RFC 4291 (February 2006, Hinden and Deering), obsoleting RFC 3513; updated by RFC 5952, 6052, 7136, 7346, 7371, and 8064[3](https://www.rfc-editor.org/info/rfc4291/) |
| Text form | Eight colon-separated 16-bit hexadecimal groups; RFC 5952 requires lowercase and a single "::" compression of the longest zero run[4](https://www.rfc-editor.org/info/rfc5952/) |
| Global unicast range | 2000::/3, the routable unicast space; link-local addresses use fe80::/10 and unique-local addresses fc00::/7[5](https://www.iana.org/assignments/iana-ipv6-special-registry) |
| Self-configuration | SLAAC (RFC 4862) lets a host combine a router-advertised prefix with a locally generated interface identifier, with no server required[6](https://www.rfc-editor.org/info/rfc4862/) |
| DNS record | AAAA resource records store IPv6 addresses, with reverse lookups under the IP6.ARPA domain (RFC 3596)[7](https://www.rfc-editor.org/info/rfc3596/) |
| Adoption | On March 28, 2026, 50% of users reached Google over IPv6 for the first time; APNIC measured about 43% of users[8](https://www.tomshardware.com/networking/ipv6-usage-reaches-historic-50-percent-across-google-services-matching-ipv4-increased-usage-eases-pressure-on-the-ipv4-address-market-as-new-protocol-designed-in-1998-finally-hits-its-stride) |
## Structure and types
IPv6 continues the IPv4 model in which a subnet prefix is associated with one link, and multiple subnet prefixes may be assigned to the same link.[3](https://www.rfc-editor.org/info/rfc4291/) An address prefix is written in CIDR-style notation, `ipv6-address/prefix-length`.[3](https://www.rfc-editor.org/info/rfc4291/) The IANA IPv6 address-space registry divides the space into large blocks: 2000::/3 is the Global Unicast range from which routable addresses are allocated to the Regional Internet Registries, ff00::/8 is multicast, fc00::/7 is Unique-Local Unicast, and fe80::/10 is Link-Local Unicast.[9](https://www.iana.org/assignments/ipv6-address-space)
Several blocks serve special purposes and are recorded in the IANA IPv6 Special-Purpose Address Registry. The loopback address ::1/128 identifies the local node itself, and the unspecified address ::/128 may appear only as a source address. The documentation prefix 2001:db8::/32, allocated in July 2004, is reserved for examples in manuals and papers, and a second documentation block, 3fff::/20, was added in July 2024. Other entries include the IPv4-mapped prefix ::ffff:0:0/96, the translation prefix 64:ff9b::/96, Teredo (2001::/32), 6to4 (2002::/16), and the Segment Routing prefix 5f00::/16.[5](https://www.iana.org/assignments/iana-ipv6-special-registry)
## Text representation
A single IPv6 address can be written in many ways. RFC 4291 allows leading zeros in each 16-bit field to be omitted, permits one run of one or more zero groups to be replaced by "::", and allows an embedded IPv4 address in the low-order 32 bits, as in `::FFFF:129.144.52.38`.[3](https://www.rfc-editor.org/info/rfc4291/) Because the "::" can appear only once, an address with two zero runs leaves a choice of where to compress, and RFC 4291 set no preference between uppercase and lowercase hexadecimal.[4](https://www.rfc-editor.org/info/rfc5952/)
**Canonical form.** RFC 5952, published in August 2010 by S. Kawamura and M. Kawashima of the IPv6 Maintenance working group, resolved this flexibility with a single recommended representation: hexadecimal characters must be lowercase, leading zeros in a field must be suppressed, and "::" must replace the longest run of zero groups, or the leftmost one if there is a tie.[4](https://www.rfc-editor.org/info/rfc5952/) For addresses with an embedded IPv4 part, such as IPv4-mapped addresses, the IPv4 portion is written in dotted decimal. In URIs, IPv6 literals are enclosed in square brackets per RFC 3986, since a bare colon would be ambiguous with the port separator.[4](https://www.rfc-editor.org/info/rfc5952/)
## How hosts obtain addresses
Stateless Address Autoconfiguration (SLAAC), specified in RFC 4862, is the mechanism by which a host configures its own addresses without a server. Routers advertise prefixes that identify the subnets on a link, and the host generates an interface identifier, in many cases derived from the interface's link-layer address, then combines the two into an address. In the absence of routers, a host can still generate link-local addresses, which are sufficient for communication among nodes on the same link.[6](https://www.rfc-editor.org/info/rfc4862/) Before an address is assigned, Duplicate Address Detection verifies its uniqueness on the link; a tentative address is not used for normal traffic while this check runs.[6](https://www.rfc-editor.org/info/rfc4862/)
Every SLAAC address carries two lifetimes. While an address is preferred, its use in new communication is unrestricted; when it becomes deprecated, new connections should use another address, but existing communication may continue, and datagrams destined to a deprecated address must still be processed. When the valid lifetime expires, the binding becomes invalid and the address may be reassigned elsewhere.[6](https://www.rfc-editor.org/info/rfc4862/) Sites that need tighter control over exact assignments can use [DHCPv6](https://www.edgepedia.org/dhcpv6) instead of, or alongside, SLAAC; hosts accept the union of information received from both sources.[6](https://www.rfc-editor.org/info/rfc4862/)
**Privacy and stability.** A stable interface identifier makes a device trackable across networks and over time. RFC 8981, published in February 2021, obsoleted RFC 4941 and specifies an extension to SLAAC that generates temporary addresses to mitigate this; it randomly generates interface identifiers as part of SLAAC privacy extensions.[10](https://www.rfc-editor.org/rfc/rfc8981.html) • [11](https://www.rfc-editor.org/rfc/rfc9099.html) The complementary need is stability within one network: RFC 8064, combined with the generation mechanism of RFC 7217, lets a node keep the same interface identifier for each network prefix, so it has a stable address on a given network but different addresses on different networks.[11](https://www.rfc-editor.org/rfc/rfc9099.html)
## DNS integration
The [Domain Name System](https://www.edgepedia.org/domain-name-system) gained IPv6 support through RFC 3596, which defines the AAAA resource record type to map a domain name to an IPv6 address and the IP6.ARPA domain for lookups based on an address, alongside updated definitions of existing query types.[7](https://www.rfc-editor.org/info/rfc3596/) The presence of an AAAA record for a zone's nameserver does not by itself guarantee IPv6-only reachability, since the full delegation chain must also resolve over IPv6.[12](https://arxiv.org/abs/2302.11393)
## Adoption
IPv6 was designed in 1998 as the successor to IPv4, whose limited address pool the IETF had already projected in 1994 would be exhausted between 2005 and 2011.[1](https://www.rfc-editor.org/info/rfc2460/) • [13](https://datatracker.ietf.org/doc/html/rfc1752) Deployment lagged for two decades, then accelerated: Google's measurement of users reaching its services over IPv6 first reached 50% on March 28, 2026, while APNIC's measurements put IPv6 use at about 43% of Internet users, with Asia and the Americas approaching 50%.[8](https://www.tomshardware.com/networking/ipv6-usage-reaches-historic-50-percent-across-google-services-matching-ipv4-increased-usage-eases-pressure-on-the-ipv4-address-market-as-new-protocol-designed-in-1998-finally-hits-its-stride) APNIC measures deployment by presenting 25 to 30 million ad impressions per day and weighting the per-economy results by Internet user counts from the UN Statistics Division and ITU-T.[14](https://blog.apnic.net/2025/11/14/how-we-measure-ipv6/) Because the two measurement systems observe traffic from different vantage points, their figures differ.[15](https://blog.apnic.net/2016/07/08/vivas-measuring-ipv6-internets-core/)
The protocol itself reached [Internet Standard](https://www.edgepedia.org/internet-standard) status as RFC 8200 (STD 86) in July 2017, again authored by Deering and Hinden, which obsoleted RFC 2460; the address format it references remains RFC 4291.[16](https://www.rfc-editor.org/info/rfc8200/)
## References
1. [RFC 2460: Internet Protocol, Version 6 (IPv6) Specification, RFC Editor](https://www.rfc-editor.org/info/rfc2460/)
2. [FAQ on IPv6 Adoption and IPv4 Exhaustion, Internet Society](https://www.internetsociety.org/deploy360/ipv6/faq/)
3. [RFC 4291: IP Version 6 Addressing Architecture, RFC Editor](https://www.rfc-editor.org/info/rfc4291/)
4. [RFC 5952: A Recommendation for IPv6 Address Text Representation, RFC Editor](https://www.rfc-editor.org/info/rfc5952/)
5. [IANA IPv6 Special-Purpose Address Registry](https://www.iana.org/assignments/iana-ipv6-special-registry)
6. [RFC 4862: IPv6 Stateless Address Autoconfiguration, RFC Editor](https://www.rfc-editor.org/info/rfc4862/)
7. [RFC 3596: DNS Extensions to Support IP Version 6, RFC Editor](https://www.rfc-editor.org/info/rfc3596/)
8. [IPv6 usage reaches historic 50% across Google services, Tom's Hardware](https://www.tomshardware.com/networking/ipv6-usage-reaches-historic-50-percent-across-google-services-matching-ipv4-increased-usage-eases-pressure-on-the-ipv4-address-market-as-new-protocol-designed-in-1998-finally-hits-its-stride)
9. [IANA IPv6 Address Space](https://www.iana.org/assignments/ipv6-address-space)
10. [RFC 8981: Temporary Address Extensions for Stateless Address Autoconfiguration in IPv6, RFC Editor](https://www.rfc-editor.org/rfc/rfc8981.html)
11. [RFC 9099: Operational Security Considerations for IPv6 Networks, RFC Editor](https://www.rfc-editor.org/rfc/rfc9099.html)
12. [How Ready Is DNS for an IPv6-Only World? arXiv](https://arxiv.org/abs/2302.11393)
13. [RFC 1752: The Recommendation for the IP Next Generation Protocol, IETF](https://datatracker.ietf.org/doc/html/rfc1752)
14. [How we measure: IPv6, APNIC Blog](https://blog.apnic.net/2025/11/14/how-we-measure-ipv6/)
15. [vizAS: Measuring IPv6 from the Internet's core, APNIC Blog](https://blog.apnic.net/2016/07/08/vivas-measuring-ipv6-internets-core/)
16. [RFC 8200: Internet Protocol, Version 6 (IPv6) Specification, RFC Editor](https://www.rfc-editor.org/info/rfc8200/)
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*Topic: Encyclopedia › Technology and the built world › Computing and digital systems › Networks and security › Networking fundamentals and architecture › Routing and addressing › IP protocol versions, variants, and options*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license