The name invalid 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 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 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, localhost

.localhost

The name localhost is reserved by the Internet Engineering Task Force in RFC 2606 as a domain name label 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 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.

Purpose

This top-level domain is sometimes used as a pseudo domain name in Uniform Resource Identifier

Uniform Resource Identifier

In computing, a uniform resource identifier is a string of characters used to identify a name or a resource on the Internet. Such identification enables interaction with representations of the resource over a network using specific protocols...

s (URIs) to convey either an error condition or in use of privacy protection. A notable instance of this usage is in the Session Initiation Protocol

Session Initiation Protocol

The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

(SIP) where the domain name anonymous.invalid in a SIP URI indicates hiding of a caller's identity.