ARIN, the American Registry for Internet Numbers, is the Regional Internet Registry (RIR) responsible for distributing and registering IPv4 addresses, IPv6 addresses and autonomous system numbers in Canada, the United States and many Caribbean and North Atlantic islands. In Internet governance, it is one of the five bodies that turn global number pools held by IANA into registered, unique allocations for network operators.
ARIN was established in 1997 as a nonprofit, member-based organization, taking over number registration for its region from the earlier InterNIC arrangements. It does not assign domain names, does not route traffic and does not regulate the Internet. Its authority rests on community-developed policy and on the shared interest of network operators in keeping the global number registry accurate.
What ARIN does as a Regional Internet Registry
ARIN performs number resource management within its service region. Its central product is the registry itself: an authoritative record of which organization holds which address block or AS number, which other operators rely on when they accept routes, investigate abuse or troubleshoot connectivity.
| Service | What it provides | Main users |
|---|---|---|
| IPv4 and IPv6 allocation | Address blocks to ISPs and end users | Network operators, enterprises |
| ASN assignment | Autonomous system numbers for BGP routing | Multihomed networks |
| Whois and RDAP | Public lookup of registration data | Operators, researchers, investigators |
| Reverse DNS delegation | in-addr.arpa and ip6.arpa zones | Address holders |
| RPKI | Cryptographic route origin authorizations | Operators securing BGP |
| Transfer services | Recording IPv4 transfers between holders | Buyers and sellers of address space |
The Resource Public Key Infrastructure has become one of ARIN’s most important services. Holders create Route Origin Authorizations (ROAs) that state which AS may announce their prefixes, and networks performing route origin validation can reject hijacked or mistaken announcements.
ARIN also operates services for the wider community, including public registration statistics, training on IPv6 deployment and the operation of authoritative servers for reverse DNS zones. These activities serve the registry’s main goal: an accurate, publicly verifiable record that lets independent networks trust one another’s address claims.
ARIN within the global registry system
The number registry is hierarchical. IANA, operated by PTI as an affiliate of ICANN, holds the global pools and allocates large blocks to the five RIRs. The RIRs then allocate to ISPs and assign to end users, and in some regions, though rarely in ARIN’s, national or local registries form an extra layer.
| Registry | Service region | Founded |
|---|---|---|
| ARIN | Canada, United States, parts of the Caribbean | 1997 |
| RIPE NCC | Europe, Middle East, parts of Central Asia | 1992 |
| APNIC | Asia-Pacific | 1993 |
| LACNIC | Latin America and parts of the Caribbean | 2002 |
| AFRINIC | Africa | 2005 |
The RIRs coordinate through the Number Resource Organization (NRO), which also acts as the Address Supporting Organization within ICANN. Global policies, such as how IANA distributes resources to RIRs, require agreement in all five regional communities.
Policy development and membership
ARIN follows a bottom-up, open policy process. Anyone, not only members, can submit a proposal, take part in the public mailing list discussion and comment at public policy meetings. An elected Advisory Council shepherds proposals through drafting and recommends them, and the Board of Trustees ratifies them and checks that the process was followed.
The resulting rules are collected in the Number Resource Policy Manual (NRPM), which sets criteria such as minimum allocation sizes, justification of need and transfer conditions. Organizations holding resources pay annual registration fees based on the size of their holdings; these fees fund operations, and ARIN does not sell addresses as property.
Address holders sign a Registration Services Agreement (RSA). Organizations with legacy space, blocks issued before ARIN existed, can sign a Legacy RSA to gain access to services such as RPKI, which led many long-standing holders to formalize their relationship with the registry.
IPv4 exhaustion and the transfer market
ARIN reached IPv4 free pool depletion in September 2015, when it could no longer satisfy ordinary requests from its inventory. Since then, organizations needing IPv4 obtain it mainly through specified transfers from existing holders, or join a waiting list that receives small blocks from returned or revoked space.
| Need | Route through ARIN | Practical note |
|---|---|---|
| IPv6 space | Direct request with modest justification | Readily available |
| Small IPv4 block | Waiting list for unmet requests | Long and uncertain wait |
| Larger IPv4 block | Market purchase recorded as a transfer | Needs-based approval of the recipient |
| AS number | Direct request | Required for independent BGP routing |
ARIN does not set or receive the sale price in a transfer; it verifies eligibility and updates the registry. Inter-RIR transfers are possible between registries with compatible policies, which created a global IPv4 market, while IPv6 remains the long-term answer to scarcity.
Common misunderstandings about ARIN
Because ARIN data appears in many troubleshooting tools, its role is frequently overestimated or misread. Keeping the boundaries clear avoids wasted effort when dealing with abuse, routing or legal questions.
- ARIN does not control routing: it registers holders, but each network decides which routes to accept.
- ARIN is not a domain registrar: domain names are handled by registries and registrars under ICANN.
- Whois records show the registrant, often an ISP, not necessarily the person using an address.
- Address space is not owned outright in the registry sense; holders receive registration rights subject to policy.
- ARIN is not a government agency, although it cooperates with law enforcement within its policies.
Whois output also needs careful reading. A block registered to an ISP may be reassigned to a customer through a SWIP record or a reallocation, so the most specific entry identifies the responsible network. Abuse reports should go to the abuse contact listed for that entry, since ARIN itself does not investigate spam, attacks or content on the networks it registers.
A plain-language summary
Every device on the Internet needs a unique number address, in the same way every house needs a unique postal address. Someone has to keep the master list so that no two networks use the same numbers. ARIN keeps that official list for North America and parts of the Caribbean.
Companies and internet providers ask ARIN for blocks of numbers. ARIN checks the request, records who received them and publishes the records so others can look them up. It is run by the community that uses the numbers, not by a government.
An example from practice
A regional internet provider in the Midwest of the United States plans to connect to two upstream carriers for redundancy. To announce its own routes with BGP, it requests an AS number from ARIN and an IPv6 allocation, both approved after it documents its network plan. Because no free IPv4 space is available, it buys a /22 block from a company that no longer needs it, and ARIN records the transfer after confirming the provider’s need.
The provider then creates Route Origin Authorizations in ARIN’s RPKI system for all its prefixes. A few months later, a misconfigured network elsewhere accidentally announces one of the provider’s prefixes. Upstream networks that perform route origin validation reject the invalid announcement, and the provider’s customers notice nothing.