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IP address

Summary

An IP address is the number that identifies a device's connection to the internet so that data can reach it. IPv4, defined in RFC 791 in September 1981, uses 32-bit addresses, about 4.3 billion in all, and IANA handed out its last free blocks on February 3, 2011. IPv6, now specified in RFC 8200, uses 128-bit addresses, and on March 28, 2026, the share of Google's users reaching it over IPv6 passed 50% for the first time. The Domain Name System links domain names to these addresses through A and AAAA records.

Web Building Encyclopedia · Foundations · Last reviewed October 22, 2026 · 10 min read

What an IP address is

An IP address is the number that identifies a device’s connection to the internet, so that data can be delivered to it. The Internet Protocol (IP) carries data in blocks called datagrams, and the header of each one holds a source address and a destination address.1

RFC 791, the 1981 standard for IP, separates three ideas: “A name indicates what we seek. An address indicates where it is. A route indicates how to get there.”1 A domain name is the name, the IP address is the address, and the routers in between work out the route.

Two versions are in use today:

  • IPv4 addresses are 32 bits long.1 They are written as four numbers from 0 to 255 separated by dots, in the style of the private range 192.168.0.0 to 192.168.255.255.2
  • IPv6 addresses are 128 bits long.3 They are written as eight groups of hexadecimal (base 16) digits separated by colons, and runs of zeros can be shortened to a double colon.4

The examples on this page, such as 192.0.2.10 and 2001:db8::10, come from blocks reserved for documentation, so they never point at a real machine.56

The Internet Assigned Numbers Authority (IANA) sits at the top of the system. Its primary role for IP addresses is to hand pools of unallocated addresses to five Regional Internet Registries (RIRs), which pass them on to internet service providers and other networks.7 The five are AFRINIC (Africa), APNIC (Asia Pacific), ARIN (Canada, the United States and some Caribbean islands), LACNIC (Latin America and some Caribbean islands) and the RIPE NCC (Europe, the Middle East and Central Asia).7

IPv4: 32 bits and the original plan

IPv4 was defined in RFC 791, published in September 1981. It was prepared by the Information Sciences Institute at the University of Southern California for the Defense Advanced Research Projects Agency (DARPA), with Jon Postel as editor.1 The header’s version field holds the number 4, which is where the name IPv4 comes from.1

The standard fixes addresses at four octets, or 32 bits.1 An octet is 8 bits, so each of the four numbers runs from 0 to 255, and 32 bits allow 2 to the power of 32 combinations: 4,294,967,296, or about 4.3 billion.

IP’s job is narrow. RFC 791 says the protocol does not provide a reliable communication facility and has no mechanisms for end-to-end reliability, flow control or sequencing, leaving those to the protocols that run on top of it.1

RFC 791 split each address into a network part and a local part, in three classes. Class A gave 7 bits to the network and 24 bits to the local address, Class B gave 14 and 16, and Class C gave 21 and 8.1 In practice an organization got either a very large network, a Class B network of about 65,000 addresses or a Class C network of 256.

The fixed classes wasted space. Class C was too small for most organizations and Class B was larger than most needed, while routing tables grew faster than routers and their operators could manage.8 Classless Inter-Domain Routing (CIDR), first developed in 1992 and published as RFC 1519 in September 1993, dropped the classes.8

CIDR writes a block as an address, a slash and the number of leading bits that identify the network, so 172.16.0.0/16 means the first 16 bits are the network part.8 RFC 4632 replaced RFC 1519 in August 2006, and it notes that CIDR was never meant to solve the long-term exhaustion of IPv4.8

Some ranges never reach the public internet. IANA’s registry reserves 127.0.0.0/8 for loopback, the address a computer uses to talk to itself, sets aside the blocks from 224 to 239 for multicast and holds 240 to 255 for future use (the old Class E).9

How IPv4 addresses ran out

On January 31, 2011, IANA allocated two blocks to APNIC. That triggered a global policy to split IANA’s remaining blocks equally among the five RIRs. On February 3, 2011, after those final allocations, the Number Resource Organization (NRO) announced that the free pool of IPv4 addresses was fully depleted.10

Each of those blocks was a /8, one 256th of all IPv4 space, or about 16.8 million addresses.10 IANA’s registry dates the last five, 102/8, 103/8, 104/8, 179/8 and 185/8, to February 2011.9 NRO chairman Raúl Echeberría said that day: “Deploying IPv6 is now a requirement, not an option.”10

The registries still had stock of their own, and each ran down on its own schedule, usually through a final phase with tight limits on how much one member could get:

DateRegistryWhat happened
April 15, 2011APNICReached its final /8. Each account holder could get at most a /22, or 1,024 addresses.11
2012RIPE NCCReached its last /8 and began handing out small /22 blocks.12
September 24, 2015ARINIssued the final IPv4 addresses in its free pool.13
November 25, 2019RIPE NCCMade its final /22 allocation and announced it had run out.14
January 13, 2020AFRINICApproved a request that left no more than a /11 in its final /8, which started phase 2 of exhaustion, with a maximum of a /22 per request.15
August 2020LACNICAnnounced that its last available IPv4 block had been reserved, after a final phase that began in February 2017.16

Since then, the registries mostly reuse addresses that are returned or recovered. ARIN fills approved requests from a waiting list, or organizations turn to the IPv4 transfer market.13 The RIPE NCC’s waiting list is open only to members that have never had an IPv4 allocation from it, and each can get a single /24, or 256 addresses.14 LACNIC reassigns recovered addresses after a six-month quarantine.16

Scarcity now shows in prices. Amazon Web Services began charging $0.005 an hour for every public IPv4 address on February 1, 2024, and said the cost of acquiring one had risen by more than 300% over the previous five years.17 That works out to about $3.65 for a 730-hour month.

Private addresses and NAT

Two techniques let many devices share a small number of public IPv4 addresses.

Private address ranges. RFC 1918, published in February 1996, reserves three blocks for use inside homes and organizations: 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255 and 192.168.0.0 to 192.168.255.255.2 Any network can reuse them, because hosts with these addresses cannot connect directly to the outside internet. Its stated aim was to conserve globally unique address space where uniqueness is not needed.2

Network address translation (NAT). A NAT device, such as a home router, rewrites addresses as traffic passes through it, so private addresses on the inside appear as public ones on the outside. RFC 1631 described NAT in May 1994 as a short-term solution to the shortage, offering temporary relief while more far-reaching solutions were worked out, and said it was not suited to the long term.18

RFC 3022, which replaced it in January 2001, also describes Network Address Port Translation (NAPT). With NAPT, many inside devices share one outside address, and each conversation is told apart by a different port number.19 This is how a laptop and several phones on one home network can all reach websites through a single public IPv4 address.

Some internet providers add a second layer, called carrier-grade NAT (CGN), shared by many customers. RFC 6598 set aside 100.64.0.0/10 in April 2012 for the links between CGN equipment and customers’ routers.20

IPv6: 128-bit addresses

The longer-term answer came from the Internet Engineering Task Force (IETF), which ran a selection process under the name IP Next Generation, or IPng. In January 1995, RFC 1752 recommended a revised proposal called Simple Internet Protocol Plus, with 128-bit addresses, as the basis for IPng.21 Its authors called 16-byte fixed-length addresses “the best compromise between efficiency, functionality, flexibility, and global applicability.”21

Stephen Deering and Robert Hinden wrote the specification. It was first published as RFC 1883 in December 1995 and revised as RFC 2460 in December 1998.2223 RFC 8200 replaced RFC 2460 in July 2017 and is a full Internet Standard, STD 86.24

The main change is size: IPv6 increases the address from 32 bits to 128 bits.3 That allows 2 to the power of 128 addresses, about 3.4 times 10 to the 38th power. The standard also drops or makes optional some IPv4 header fields, and leaves fragmentation (splitting packets that are too big) to the sending device instead of routers along the path.3

An IPv6 address is written as eight groups of up to four hexadecimal digits separated by colons. A double colon stands for one run of zero groups and can appear only once, and the loopback address is ::1.4 RFC 5952, from August 2010, adds that leading zeros must be dropped and letters written in lowercase, so 2001:db8:0:0:1:0:0:1 becomes 2001:db8::1:0:0:1.25

Moving to IPv6 does not switch IPv4 off. RFC 4213 describes dual stack, in which hosts and routers carry complete support for both protocols at once.26

Adoption

IPv6 address space has been available from the registries since 1999, the NRO noted in 2011.10 World IPv6 Day, a one-day trial, took place on June 8, 2011. The Internet Society then organized World IPv6 Launch on June 6, 2012, when internet providers, home router makers and web companies committed to enabling IPv6 permanently.27

Google continuously measures how many of its users reach it over IPv6, and has recorded the figure since 2008.2829 In an April 21, 2026 report on Google’s data, the Internet Society said the share had passed 50% for the first time on March 28, 2026, at 50.10%. It had come close at 49.56% on June 21, 2025, and had stayed just below 50% in the months before the crossing.29 Adoption is uneven: the same report, citing Internet Society Pulse data, lists France at 73% and India at 72%, against Spain at 10% and Egypt at 4%.29

Websites trail their visitors. As of October 2026, W3Techs finds that 32.1% of websites support IPv6, counting a site if a 128-bit address is assigned to it.30

From domain names to IP addresses

People rarely type IP addresses. The Domain Name System (DNS) stores the link between a name and its addresses in records:

  • A record. Defined in RFC 1035 in November 1987, it holds one 32-bit IPv4 address for a host.31
  • AAAA record. Defined in RFC 3596 in October 2003, it holds one 128-bit IPv6 address and has record type number 28.32
  • PTR record. Reverse lookups go from address to name. They use the special domain in-addr.arpa for IPv4 and ip6.arpa for IPv6, with the address written backward, so 10.2.0.52 is looked up as 52.0.2.10.in-addr.arpa.3132

A name can have several A and AAAA records at once. Cloudflare’s documentation describes both types as mapping a domain name to one or more IPv4 or IPv6 addresses.33 GitHub Pages, for example, lists four A records and four AAAA records for a custom domain, and tells owners who add the AAAA records to keep the A records as well.34

When a browser gets both kinds of address, it has to pick one. The method known as Happy Eyeballs, now RFC 8305 from December 2017, prefers IPv6 but staggers connection attempts across both protocols and keeps whichever connects first, with a recommended default delay of 250 milliseconds between attempts.35 The aim is that a broken or slow IPv6 path does not leave the visitor waiting.35

Shared and dedicated IP addresses

One IP address can serve many websites. With name-based virtual hosting, the web server reads the host name that the browser sends in each HTTP request and picks the matching site. The Apache documentation puts it this way: “many different hosts can share the same IP address.”36 The older method, IP-based virtual hosting, needs a separate address for each site.36

Encrypted connections once complicated sharing, because the server has to choose a certificate before any HTTP request arrives. Server Name Indication (SNI), a TLS extension first defined in RFC 3546 in June 2003 and now specified in RFC 6066, lets the browser name the site at the start of the TLS connection. Its stated purpose is secure connections to servers that host multiple virtual servers at a single network address.3738

The Apache documentation now says the historical, client-support reasons for IP-based hosting no longer apply to a general-purpose web server, and recommends name-based hosting unless equipment explicitly demands otherwise.36

Timeline

  1. IPv6 passes 50% of Google users for the first time, at 50.10%29
  2. LACNIC announces that its last available IPv4 block has been reserved16
  3. The RIPE NCC makes its final /22 allocation and runs out of IPv414
  4. RFC 8200 replaces RFC 2460 as the IPv6 standard (STD 86)24
  5. ARIN issues the final addresses in its IPv4 free pool13
  6. World IPv6 Launch: providers and web companies enable IPv6 permanently27
  7. RFC 6598 reserves 100.64.0.0/10 for carrier-grade NAT20
  8. APNIC reaches its final /8 of IPv4 addresses11
  9. IANA's free pool of IPv4 addresses is fully depleted10
  10. RFC 2460 revises the IPv6 specification23
  11. RFC 1918 reserves private IPv4 address ranges2
  12. RFC 1883 publishes the first IPv6 specification22
  13. RFC 1631 describes network address translation (NAT)18
  14. RFC 1519 introduces Classless Inter-Domain Routing (CIDR)8
  15. RFC 791 defines IPv4 with 32-bit addresses1

What it means for your website

You rarely need to know your site’s IP address, but you meet it every time you connect a domain or change hosts.

  • Enter A and AAAA records exactly as given. When you connect a domain, your host or builder gives you either a host name for a CNAME record or IP addresses for A and AAAA records. GitHub Pages, for example, publishes four of each.34 Delete old A records that point at a previous host. Our DNS glossary entry explains the record types.
  • Expect new addresses when you move. A new host means new IP addresses but the same domain, so updating the A and AAAA records is the step that sends visitors to the new server. GitHub warns that DNS changes can take up to 24 hours to spread.34 Build that into any website migration.
  • Add IPv6 if your host offers it. Google’s share of users arriving over IPv6 passed 50% for the first time on March 28, 2026, while about a third of websites had an IPv6 address as of October 2026.2930 Keep the A record too, as GitHub advises.34
  • A shared IP address is normal. Name-based hosting lets many sites share one address, and SNI lets each have its own HTTPS certificate.3638 The Apache documentation reserves IP-based hosting for equipment that explicitly demands it.36
  • Know what a proxy shows. With Cloudflare proxying turned on, lookups return Cloudflare’s shared anycast addresses instead of your server’s, which hides your origin. Records set to DNS only show your server’s real address to anyone who looks.39
  • Count IPv4 in cloud server costs. On AWS, each public IPv4 address, in use or idle, has cost $0.005 an hour since February 1, 2024, and the rate was unchanged as of October 2026.1740

Our guide to how a website works shows where addresses fit in a page load, and domains and hosting covers what to ask a host before you sign up.

See also

References

  1. RFC 791: Internet Protocol. Jon Postel (ed.), Information Sciences Institute, for DARPA, RFC Editor. 1981-09. accessed October 22, 2026.
  2. RFC 1918: Address Allocation for Private Internets (BCP 5). Y. Rekhter, B. Moskowitz, D. Karrenberg, G. J. de Groot and E. Lear, RFC Editor. 1996-02. accessed October 22, 2026.
  3. RFC 8200: Internet Protocol, Version 6 (IPv6) Specification. S. Deering and R. Hinden, Internet Engineering Task Force, RFC Editor. 2017-07. accessed October 22, 2026.
  4. RFC 4291: IP Version 6 Addressing Architecture. R. Hinden and S. Deering, Internet Engineering Task Force, RFC Editor. 2006-02. accessed October 22, 2026.
  5. RFC 5737: IPv4 Address Blocks Reserved for Documentation. Internet Engineering Task Force, RFC Editor. 2010-01. accessed October 22, 2026.
  6. RFC 3849: IPv6 Address Prefix Reserved for Documentation. Internet Engineering Task Force, RFC Editor. 2004-07. accessed October 22, 2026.
  7. Number Resources. Internet Assigned Numbers Authority (IANA). accessed October 22, 2026.
  8. RFC 4632: Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan (BCP 122). V. Fuller and T. Li, Internet Engineering Task Force, RFC Editor. 2006-08. accessed October 22, 2026.
  9. IANA IPv4 Address Space Registry. Internet Assigned Numbers Authority (IANA). 2025-10-10. accessed October 22, 2026.
  10. IPv6 adoption at critical phase (free pool of IPv4 addresses fully depleted). Number Resource Organization. 2011-02-03. accessed October 22, 2026.
  11. APNIC IPv4 Address Pool Reaches Final /8. APNIC, apnic-announce mailing list. 2011-04-15. accessed October 22, 2026.
  12. The RIPE NCC has run out of IPv4 Addresses. RIPE NCC. accessed October 22, 2026.
  13. ARIN IPv4 Free Pool Reaches Zero. American Registry for Internet Numbers (ARIN). 2015-09-24. accessed October 22, 2026.
  14. The RIPE NCC has run out of IPv4 Addresses (ripe-list announcement). Nikolas Pediaditis, RIPE NCC, ripe-list mailing list. 2019-11-25. accessed October 22, 2026.
  15. AFRINIC enters IPv4 Exhaustion Phase 2. AFRINIC. accessed October 22, 2026.
  16. IPv4 Exhaustion: LACNIC Has Assigned the Last Remaining Address Block. LACNIC Blog. 2020-08-28. accessed October 22, 2026.
  17. New: AWS Public IPv4 Address Charge + Public IP Insights. AWS News Blog, Amazon Web Services. 2023-07-28. accessed October 22, 2026.
  18. RFC 1631: The IP Network Address Translator (NAT). K. Egevang and P. Francis, RFC Editor. 1994-05. accessed October 22, 2026.
  19. RFC 3022: Traditional IP Network Address Translator (Traditional NAT). P. Srisuresh and K. Egevang, RFC Editor. 2001-01. accessed October 22, 2026.
  20. RFC 6598: IANA-Reserved IPv4 Prefix for Shared Address Space (BCP 153). Internet Engineering Task Force, RFC Editor. 2012-04. accessed October 22, 2026.
  21. RFC 1752: The Recommendation for the IP Next Generation Protocol. S. Bradner and A. Mankin, IPng Area Directors, RFC Editor. 1995-01. accessed October 22, 2026.
  22. RFC 1883: Internet Protocol, Version 6 (IPv6) Specification. S. Deering and R. Hinden, RFC Editor. 1995-12. accessed October 22, 2026.
  23. RFC 2460: Internet Protocol, Version 6 (IPv6) Specification. S. Deering and R. Hinden, RFC Editor. 1998-12. accessed October 22, 2026.
  24. Information on RFC 8200 (STD 86). RFC Editor. accessed October 22, 2026.
  25. RFC 5952: A Recommendation for IPv6 Address Text Representation. Internet Engineering Task Force, RFC Editor. 2010-08. accessed October 22, 2026.
  26. RFC 4213: Basic Transition Mechanisms for IPv6 Hosts and Routers. Internet Engineering Task Force, RFC Editor. 2005-10. accessed October 22, 2026.
  27. World IPv6 Launch. Internet Society. accessed October 22, 2026.
  28. IPv6: Statistics. Google. accessed October 22, 2026.
  29. 18 Years Later, IPv6 Reaches Majority. Mat Ford, Internet Society Pulse. 2026-04-21. accessed October 22, 2026.
  30. Usage statistics of IPv6 for websites. W3Techs. 2026-10. accessed October 22, 2026.
  31. RFC 1035: Domain Names, Implementation and Specification. P. Mockapetris, Information Sciences Institute, RFC Editor. 1987-11. accessed October 22, 2026.
  32. RFC 3596: DNS Extensions to Support IP Version 6. S. Thomson, C. Huitema, V. Ksinant and M. Souissi, RFC Editor. 2003-10. accessed October 22, 2026.
  33. DNS record types. Cloudflare DNS documentation. accessed October 22, 2026.
  34. Managing a custom domain for your GitHub Pages site. GitHub Docs. accessed October 22, 2026.
  35. RFC 8305: Happy Eyeballs Version 2: Better Connectivity Using Concurrency. Internet Engineering Task Force, RFC Editor. 2017-12. accessed October 22, 2026.
  36. Name-based Virtual Host Support. Apache HTTP Server Version 2.4 Documentation, The Apache Software Foundation. accessed October 22, 2026.
  37. RFC 3546: Transport Layer Security (TLS) Extensions. Internet Engineering Task Force, RFC Editor. 2003-06. accessed October 22, 2026.
  38. RFC 6066: Transport Layer Security (TLS) Extensions: Extension Definitions. D. Eastlake 3rd, Internet Engineering Task Force, RFC Editor. 2011-01. accessed October 22, 2026.
  39. Proxy status. Cloudflare DNS documentation. accessed October 22, 2026.
  40. Amazon VPC pricing. Amazon Web Services. accessed October 22, 2026.

Web Building Encyclopedia. First published October 22, 2026. Last reviewed October 22, 2026. Every fact on this page is linked to a source we opened. Spotted an error? Write to [email protected].