Localhost is the name every computer uses to refer to itself. When a developer opens http://localhost:3000 in a browser, the request never leaves the machine: the browser talks to a program running on that same computer. It is how websites are built and tested privately before they go live.
How localhost loops back to your own machine
The name localhost is reserved and resolves to the loopback address, 127.0.0.1 in IPv4 and ::1 in IPv6. In fact the whole 127.0.0.0/8 range is set aside for loopback. Traffic sent there is handled entirely by the operating system's network stack and never reaches the network card, router or internet. Because nothing outside can reach it by default, it is a safe place to run unfinished code.
The number after the colon is the port, which picks which program on your computer gets the request. Development tools pick their own defaults: Vite uses 5173, many Node and React tools use 3000, and Django uses 8000.
A typical development example
A developer working on a café's new site runs npm run dev. The tool starts a small web server and prints a link like http://localhost:5173. Every time a file is saved, the page in the browser reloads with the change. Only when the site is finished does it go through deployment to a real server with a public domain.
Why it matters to someone who is not a developer
If a freelancer sends you a localhost link to review, it will not work on your computer: it points to your own machine, not theirs. Ask for a preview or staging URL instead. Likewise, a live site that still contains links or images pointing to localhost will look broken to everyone except the developer.
Frequent localhost problems
- "Refused to connect". The development server is not running, or it is on a different port than the one you typed.
- "Port already in use". Another program, often an earlier copy of the same server, is holding that port.
- Testing from a phone with localhost. On the phone, localhost means the phone. Use the computer's local network address instead, if the tool allows it.
- Hard-coded localhost addresses left in production code, such as an API URL that was never switched to the live one.
Related terms
- Port: the number after localhost that selects a program.
- IP address: 127.0.0.1 is the special one localhost maps to.
- Deployment: moving a site from localhost to the internet.
- Server: what you are running locally.