An IP address, short for Internet Protocol address, is a numerical label for a device on a network that uses the Internet Protocol. IPv4 (Internet Protocol version 4) uses 32-bit addresses, and IPv6 (Internet Protocol version 6) uses 128-bit addresses. A DNS name such as example.com points to an IP address.
History of IP address
Creator: Internet Engineering Task Force (IETF). Year: 1981.
An IP address is a number that names a device on a network. It tells other devices where to send data. The address does two jobs. It identifies the network interface of a device. It also gives the location of that device, so that a path can be set up to it. Each IP packet carries the sender address and the receiver address in its header. The Internet Protocol (IP) is the set of rules that moves data between these devices.
The Internet Engineering Task Force (IETF) published RFC 791 in September 1981. RFC means Request for Comments, a numbered series of Internet standards. This version is IPv4. It was first used on the ARPANET, the early research network that came before the Internet, in 1983. IPv4 addresses are 32 bits long. That gives 4,294,967,296 possible addresses. In 1981, the rules changed to a class system with classes A, B and C. In September 1993, Classless Inter-Domain Routing (CIDR) replaced the class system.
By the early 1990s, the supply of IPv4 addresses ran low. The IETF began work on a new version. The result was IPv6, with 128-bit addresses. The IPv6 standard RFC 2460 was published in December 1998. RFC 8200 replaced it in July 2017. The American Registry for Internet Numbers (ARIN) depleted its free pool of IPv4 addresses on 24 September 2015. As of 2026, IPv6 use exceeds 50 percent worldwide.1
RFC 1918, published in February 1996, reserves address ranges for private networks. Those addresses need not be unique on the public Internet. Homes and offices use them often. Regional registries assign blocks to internet service providers (ISPs). ISPs then give addresses to their customers.
Changelog and latest update
The core IPv4 format has stayed the same since 1981. Address rules changed in steps. In September 1993, RFC 1519 introduced CIDR. CIDR writes an address with a slash and a prefix length, such as /24. RFC 4632 replaced RFC 1519 in August 2006. It sets the CIDR address assignment and aggregation plan.
RFC 1918 followed in February 1996 and set aside private ranges. In December 1998, RFC 2460 defined IPv6. In July 2017, RFC 8200 obsoleted RFC 2460. RFC 8200 is the current IPv6 specification. Its 128-bit address size replaced the 32-bit IPv4 size for new networks that use IPv6.
In 2015, ARIN reached the end of its free IPv4 pool. The depletion date was 24 September 2015. After that date, organizations get IPv4 blocks through a waiting list or through transfers from other holders. IPv4 and IPv6 are both still in use side by side.
Latest update: IANA IPv4 Address Space Registry, 2025-10-10
The IANA IPv4 Address Space Registry was last updated on 10 October 2025. It lists how the Internet Assigned Numbers Authority (IANA) assigns IPv4 blocks to the five regional internet registries. Each entry shows the block and its allocation status.
Usage examples
An IP address appears in network settings, server logs, firewall rules and DNS records. Administrators use it to allow or block a device. Web hosts assign an IP address to each server. The examples below show a Linux command, a subnet size, private ranges, an Apache rule and a curl option.
# list IPv4 and IPv6 addresses
ip -brief address showThis command lists each network interface on a Linux machine with its IP addresses. Use it to find the address of a server or workstation before you set a firewall rule or a DNS record. The brief option prints one short line for each interface.
192.0.2.1/24
prefix bits: 24
host bits: 8
addresses: 256
subnet mask: 255.255.255.0The /24 suffix means the first 24 bits name the network. The last 8 bits name the hosts. Eight host bits give 256 addresses. The same network written as a subnet mask is 255.255.255.0.
10.0.0.0/8
172.16.0.0/12
192.168.0.0/16These three blocks are reserved for private networks by RFC 1918. Each site can use them inside its own network. Addresses in these ranges are not meant for the public Internet and do not need to be unique worldwide.
<RequireAll>
Require all granted
Require not ip 203.0.113.25
</RequireAll>Apache 2.4 uses the Require directive to control access. This rule grants access to every client except 203.0.113.25. Place it in an .htaccess file or in the server configuration for the site.
# request the page over IPv6 only
curl -6 -I https://example.comThe -6 option tells curl to connect with IPv6. The -I option asks only for the response headers. The command fails if the host has no IPv6 address. Use -4 instead to force an IPv4 connection.
References
These sources are outside Websnoogie and were checked on 2026-10-12.
IP address
Wikipedia
Encyclopedia article on IP address functions, IPv4 and IPv6 versions, CIDR notation, subnets, private address ranges and how IANA and the regional registries manage address space.
RFC 791: Internet Protocol
RFC Editor
The original Internet Protocol standard, published September 1981. It defines the packet header, the 32-bit address format and the rules for sending datagrams between hosts on connected networks.
RFC 8200: IPv6 Specification
RFC Editor
The Internet Protocol version 6 specification, dated July 2017. It obsoletes RFC 2460 and defines the 128-bit address format and the IPv6 packet header used by current IPv6 networks.
RFC 1918: Address Allocation for Private Internets
RFC Editor
Best Current Practice from February 1996. It reserves address ranges for private networks, which are not routed on the public Internet and need not be unique worldwide.
RFC 4632: CIDR Address Plan
RFC Editor
Best Current Practice from August 2006 describing Classless Inter-domain Routing (CIDR) and its address assignment and aggregation plan. It obsoletes RFC 1519, which introduced CIDR in September 1993.
IPv4 Address Space Registry
IANA
Official registry of IPv4 address blocks assigned by IANA to the regional internet registries. The page was last updated on 10 October 2025 and shows the allocation status of each block.
IPv4 Address Space Guide
American Registry for Internet Numbers (ARIN)
ARIN guide to IPv4 addresses. It states that ARIN's free pool was depleted on 24 September 2015 and describes the waiting list and transfers used after depletion.
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