localhost

basicwidespread

Localhost is the name your computer uses to refer to itself: if you enter this address, your machine connects to itself, not to the internet.

In plain terms

The term localhost literally means 'local host'. It is like writing a letter to yourself and putting it in your own mailbox. The message never leaves the house, yet it arrives just like any other letter.

Developers test upcoming websites and programs at this address before anyone else can see them. Your machine acts as both the server and the visitor at this time, so no internet connection is needed for the test.

In the AI world, you encounter it more frequently because language models running on your own machine and trial pages launched by coding agents are accessible via localhost addresses. If an agent states that "the application is running on localhost:3000", the result can only be viewed on the machine where the agent is working. This is usually your own machine, but if the agent runs in the cloud, the address points to that machine, and it will not open in your browser.

A regular web address points to the same place in the world for everyone. Localhost, however, refers to each person's own machine, so sending such a link to someone else is useless.

How it works

A program starts on your machine (for example, a web server for a site under construction or a local AI model) and begins listening on a numbered "door," or port.
You enter the address with the port number into the browser, for example: localhost:3000.
Your computer translates the localhost name to a special numeric address pointing to itself. In traditional addressing (IPv4), this is usually 127.0.0.1, and in the newer addressing system (IPv6), it is ::1.
The operating system sees that this address points to itself, so it does not send the request out over the network cable or Wi-Fi but loops it back internally. This is called loopback.
The program listening on the port receives the request and responds, so you see the page in your browser just as if it came from the internet.

An everyday example

You ask a coding agent to create a simple appointment booking page for your hair salon. A few minutes later, it writes: 'Done, open the http://localhost:5173 address.' The page appears in your browser, but your guests can only access it if it is also uploaded to a real hosting server.

Who is affected

developerA daily work tool, because every application, database, and local AI model being developed is first tested at this address. → Code

entrepreneur, executiveIf you have a website or application built with an AI tool, you must know that the version running on localhost is not yet public and not a finished product.

parent, student, teacherWhen learning programming, your first personal website almost certainly appears at a localhost address. → Education & Family

everyoneAnyone running an AI model or agent on their own machine will usually find the interface at a localhost address.

A common misconception

MythWhat runs on localhost is already on the internet, and its link can be sent to anyone.

In realityLocalhost always means the machine of whoever opens it, so for anyone else the link returns an error or shows a completely different program running on their own machine. To make the application public, it must be deployed to a server.

Limits and risks

A program running on localhost is only accessible as long as your computer is on and the program is running.
'Local' does not automatically mean secure: a malicious website may try to reach unprotected local services running on your machine through your browser.
A single misconfiguration can make a service intended for local use accessible across the entire network, which is especially risky on public Wi-Fi.
What works flawlessly on your own machine may still break in production because the load, settings, and data are different there.

Similar, but not the same

szerver – A server is a machine or program that serves others, while localhost is just an address used to access a server running on your own machine.
local network – On a local network, home or office devices can see each other, but localhost is accessible only by the machine itself.
production environment – In a live environment, real users use the application, while on localhost only the creator tests it.

Related concepts

Broader

computer network, development environment, IP address

Narrower

feedback address, local developer server

On the edge

port, szerver, helyben futtatott modell, installation, production environment, coding agent

A brief history

The address range starting with 127, pointing back to the local machine, was reserved for this purpose by early internet standards in the 1980s. The name localhost has been used by computers for decades. As a reserved name, it was designated by the 1999 RFC 2606 standard, and the 2013 RFC 6761 confirmed that it must always point to the local machine.

Where things stand

For decades, it has been a default component of every computer and operating system. By the end of 2026, due to locally running AI models and coding agents, non-IT professionals will encounter it increasingly often.

Status: 2026 November

Also known as

localhost · local host · 127.0.0.1 · ::1 · loopback · feedback address · local machine · local host

Sources

IETF RFC 6761: Special-Use Domain Names

This entry was machine-edited and has undergone machine expert review; the “In the news” section updates automatically from Agent Age stories.