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A “special domain name” is properly called a special-use domain name: a DNS name that internet standards reserve for protocol-level special treatment, so software may handle it differently from an ordinary registered name. Examples include localhost., invalid., test. and example.com.. The term comes from RFC 6761, and the official list is the IANA Special-Use Domain Names registry.
What the standard says
RFC 6761, published by the IETF in February 2013 (authors Stuart Cheshire and Marc Krochmal), states in its abstract that it “describes what it means to say that a Domain Name (DNS name) is reserved for special use, when reserving such a name is appropriate, and the procedure for doing so.” It also “establishes an IANA registry for such domain names, and seeds it with entries for some of the already established special domain names.”
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The point is to separate rules that must work the same way across networks from ordinary name allocation on the public Internet. If declaring a name special would not change how implementations behave, RFC 6761 says normal DNS registration mechanisms may be more appropriate. Special-use reservation is not a way around the usual registration process.
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Does it cover subdomains?
Yes. IANA states that the special-use designation applies to each listed domain and its subdomains. Reserving test. therefore covers anything.test. as well.
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Examples and why behavior differs by name
“Special-use” is not one uniform rule. Each name has its own treatment, defined in its own specification.
localhost.: users may assume IPv4 and IPv6 address queries resolve to the respective loopback address. Name-resolution APIs and libraries should return loopback addresses for address queries and negative responses for other query types.invalid.: users may assume queries return NXDOMAIN, because the name is specified as nonexistent.test.: reserved, with its subdomains, for testing. Default responses are negative, but administrators may configure different behavior on private networks.example.,example.com.,example.net.,example.org.: reserved, with their subdomains, for documentation.- Later additions: the current registry also lists names such as
home.arpa.,onion.andlocal., each linked to its own RFC. Take their behavior from those documents rather than inferring it from the label “special-use.”
How a name becomes special-use
A proposed name should support functionality that genuinely needs special handling. The specification must go through an IETF Standards Action or IESG Approval, which is the procedure IANA’s registry lists. It must say how implementations recognize names covered by the rule, and it must address seven areas:
- Users
- Application software
- Name-resolution APIs and libraries
- Caching DNS servers
- Authoritative DNS servers
- DNS server operators
- Registries and registrars
How to check whether a name is reserved
- Open the IANA Special-Use Domain Names registry. The page showed a last-updated date of 2026-05-22 when reviewed.
- Look for the name or any parent suffix of it, since subdomains are covered.
- Follow the entry’s reference to its RFC and read what it says about resolvers, servers, private-network overrides and registration.
When comparing two special-use names, useful axes are: the defining RFC, whether the rule covers subdomains, expected resolver and server behavior, whether private networks may override the defaults, and whether registries or registrars can allocate the name.
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