The name localhost is reserved by the Internet Engineering Task Force

Internet Engineering Task Force

The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...

 (IETF) in RFC 2606 (June 1999) as a domain name label that may not be installed as a top-level domain

Top-level domain

A top-level domain is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a...

 in the Domain Name System

Domain name system

The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...

 (DNS) of the Internet

Internet

The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

.

Reserved DNS names

In 1999, the Internet Engineering Task Force

Internet Engineering Task Force

The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...

 reserved the DNS labels localhost, example

.example

The name example is reserved by the Internet Engineering Task Force in RFC 2606 as a domain name that may not be installed as a top-level domain in the Domain Name System of the Internet.-Reserved DNS names:...

, invalid

.invalid

The name invalid is reserved by the Internet Engineering Task Force in RFC 2606 as a domain name that may not be installed as a top-level domain in the Domain Name System of the Internet.-Reserved DNS names:...

, and test

.test

The name test is reserved by the Internet Engineering Task Force in RFC 2606 as a domain name that is not intended to be installed as a top-level domain in the global Domain Name System of the Internet for production use....

so that they may not be installed into the root zone

DNS root zone

A DNS root zone is the top-level DNS zone in a Domain Name System hierarchy. Most commonly it refers to the root zone of the largest global DNS, deployed for the Internet. Ultimate authority over the DNS root zone rests with the US Department of Commerce NTIA...

 of the Domain Name System.

The reasons for reservation of these top-level domain names is to reduce the likelihood of conflict and confusion. This allows the use of these names for either documentation purposes or in local testing scenarios.

Conventional use

The name localhost

Localhost

In computer networking, localhost is the standard hostname given to the address of the loopback network interface. The name is also a reserved top-level domain name In computer networking, localhost (meaning this computer) is the standard hostname given to the address of the loopback network...

 is a commonly defined hostname for the loopback

Loopback

Loopback describes ways of routing electronic signals, digital data streams, or flows of items from their originating facility back to the source without intentional processing or modification...

 interface in most TCP/IP systems, resolving to the IP address

IP address

An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...

es 127.0.0.1 in IPv4

IPv4

Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...

 and ::1 for IPv6

IPv6

Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...

. As a top-level domain the name has traditionally been defined statically in host DNS implementations with address records (A and AAAA) pointing to the same loopback addresses. Any other use conflicts with widely deployed algorithms relying on this convention.