name collision: definition

A name collision happens when a name meant for one naming system, often a private company network, gets an answer from another, usually the public DNS. Traffic can then fail or go somewhere nobody intended. The risk appears whenever a new name is added to the internet, so ICANN checks every applied-for TLD top-level domain The last part of a domain name, after the final dot. Full definition of TLD for it.

What a name collision is

ICANN’s 2026 Applicant Guidebook The rulebook for applying for a new gTLD in an ICANN round. Full definition of Applicant Guidebook (AGB) defines it as a name meant to be resolved in one naming system being resolved by mistake in another, which can disrupt or redirect communication. The classic case is a user who means to reach a resource on a private network and unknowingly reaches the same name in the public DNS.

The risk is not limited to top-level names: ICANN says any new gTLD A generic top-level domain added to the internet through ICANN's New gTLD Program. Full definition of new gTLD, ccTLD country code top-level domain A top-level domain for a country or territory, usually two letters long. Full definition of ccTLD or second-level name can create it. The SSAC Security and Stability Advisory Committee ICANN's expert committee on security and stability of naming and addressing. Full definition of SSAC’s Name Collision Analysis Project (NCAP) says the problem has been known for decades, “possibly as early as the late 1980s”.

Why it happens: private names and new TLDs

Many organisations gave their internal networks an ending that did not exist in the public DNS. Queries for those names leak out to the public root. Once the same string is delegated as a new gTLD, the leaked queries start getting real answers.

Search lists add to the problem: a device appends suffixes from a list to a short name, one by one, until one works. NCAP counts “perhaps a dozen or more” root causes.

In the 2012 round, of about 1,400 applied-for string The characters an applicant asks ICANN to approve as a new top-level domain. Full definition of applied-for string, three were judged high-risk: .corp, .home and .mail. ICANN deferred them indefinitely in 2014, and on 7 September 2024 the ICANN Board ICANN's board of directors, which adopts policies and approves key decisions. Full definition of ICANN Board declined to have them reassessed.

Reserved and special-use names

Two separate systems keep names out of the root.

The IETF Internet Engineering Task Force The open body that writes internet standards such as DNS, EPP and RDAP. Full definition of IETF reserves special-use domain names Names reserved for technical uses and not delegated like normal domains. Full definition of special-use domain names under RFC Request for Comments A numbered document in the series recording the internet's technical standards and practices. Full definition of RFC 6761, in a registry The central database and system of a top-level domain, or loosely the organization that runs it. Full definition of registry run by IANA Internet Assigned Numbers Authority The functions that coordinate the root zone, IP addresses and protocol numbers. Full definition of IANA. Top-level entries include .test, .example, .invalid, .localhost, .local, .onion The reserved ending for Tor network services, kept outside the DNS. Full definition of .onion for the Tor network and .alt A reserved ending for names used outside the DNS. Full definition of .alt for naming systems outside the DNS Domain Name System The internet's directory that links domain names to computer addresses. Full definition of DNS. Some names under .arpa An infrastructure-only top-level domain, best known for holding reverse DNS records. Full definition of .arpa are listed too, such as home.arpa for home networks. The .arpa domain itself is an infrastructure TLD run by IANA, not a special-use name as a whole.
ICANN keeps a Blocked Names list in the AGB. It includes every special-use name plus strings such as .internal, which the Board reserved permanently for private use on 29 July 2024, after SSAC advice. Blocked strings cannot be applied for in any round.

RFC 8244 (October 2017) notes that no formal process coordinates the two, and that neither body can stop a third party from simply using a name.

How ICANN manages collision risk

Name collisions in the 2012 round

Under the framework approved on 30 July 2014, each new registry ran at least 90 days of continuous controlled interruption. Its zone answered queries with the address 127.0.53.53, which opens no connection, and a text record saying “Your DNS configuration needs immediate attention”. NCAP later found few reports, and only one needed action by a registry.

Name collisions in the 2026 Round

On 7 September 2024 the Board approved the Name Collision Risk Management framework, which replaces the 2014 one.

Initial Assessment. After String Confirmation Day, an assessment overseen by ICANN’s Technical Review Team (TRT) checks each string using root server A server that answers for the DNS root zone and points to TLD servers. Full definition of root server and resolver The DNS server that looks up domain names on behalf of users. Full definition of resolver logs, among other data. Its report goes to public comment An open period when anyone can comment on ICANN drafts. Full definition of public comment.
Temporary Delegation. Strings not found high-risk are delegated to name server A server that holds a domain's DNS records and answers lookups. Full definition of name server run by ICANN to measure real traffic, for at least 90 and at most 365 days (as of October 2026). The root grows by no more than about five percent a month, roughly 75 delegations a month at first (as of October 2026). contracting The stage when a successful applicant signs its Registry Agreement with ICANN. Full definition of contracting waits until this step ends.
High-risk strings. As of October 2026, they go on a Collision String List. Within 90 days of that designation (or of contention resolution, if the string is in contention), the applicant An organization that has applied to ICANN to run a new gTLD. Full definition of applicant may withdraw for a full refund of the evaluation fee, or file a Mitigation Plan (up to 180 days on request) whose actions take at most two years. Its review fee is estimated at USD 100,000 to USD 150,000. If the plan fails, the application is terminated, subject to a challenge within 21 days. The refund excludes .corp, .home and .mail.

In March 2026, after public comment, ICANN decided not to use controlled interruption over IPv6, version 6 of the IP Internet Protocol The internet's basic rules for sending data, using numeric addresses for every connected device. Full definition of IP, in the 2026 Round. ICANN considers it unlikely that collisions will affect significant numbers of networks or users, but the risk remains.

What network administrators should do

ICANN’s guide for IT professionals recommends:

Treat 127.0.53.53 in a log as a collision warning.
Use fully qualified domain name A readable internet name made of labels separated by dots. Full definition of domain name (FQDN Fully Qualified Domain Name A complete domain name including every part up to the top-level domain. Full definition of FQDN) everywhere and stop relying on search lists.
Move private names under a registered domain, and monitor until the old names are no longer used.
Without a registered domain, use only reserved options: .internal, home.arpa for home networks, or .test for testing.
Report a collision to ICANN where there is a reasonable belief of demonstrable, severe harm.

An example: a company’s laptops have corp.example.com in their search list, so typing “intranet” reaches intranet.corp.example.com, a name the company controls. Had the company invented its own ending, those queries would leak to the root, and once that string became a gTLD generic top-level domain A top-level domain not tied to a country, run under ICANN contracts. Full definition of gTLD under controlled interruption, users would get 127.0.53.53 instead.

name collision