Localhost is the loopback destination for the device you are using. When your browser opens a localhost URL, it connects back to your own computer—not to a public website or another device on your Wi-Fi. Developers use a local web server on localhost to build and test applications before deploying them elsewhere.
What localhost means
Localhost is a familiar name for a loopback destination: a request addressed to it is sent back to the same device that made the request. The IPv4 loopback address 127.0.0.1 is commonly used for this purpose. See MDN’s localhost definition.
This makes localhost useful for development: a web server can run on your own machine, and your browser can connect to it without making the site publicly available. The server and browser may be separate programs, but the request stays on the same device.
How a localhost URL works
In http://localhost:8000, localhost is the host and 8000 is the port. The host identifies the destination machine; the port identifies the service endpoint on that machine. The server must be running and listening on the address and port in the URL, or the browser will not reach it. MDN uses localhost:8000 as an example in its local testing server guide.
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The port is not a permanent property of localhost. A development server may choose a default, let you configure one, or select another available port if its usual one is occupied. Use the exact local URL reported by the server.
Localhost versus a local-network address
Loopback and local-network addresses serve different purposes. A request to localhost or 127.0.0.1 targets the same device. An address such as 192.168.0.1 identifies a host on a local network, such as a router; it is not another spelling of localhost. MDN distinguishes loopback from local-network addresses in its localhost documentation.
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Consequently, a service listening only on loopback is generally for use on that machine. A different device on the network cannot use its own localhost address to reach your computer: that would point back to itself. Access from another device requires the server to be configured to accept network connections and the other device to use the computer’s reachable network address. Do not expose a development server to a network unless that is intended and appropriate for your environment.
Run a local website for testing
Prefer the development server provided by your framework or language; it may handle project-specific behavior that a generic file server does not. If you only need to serve static files and have Python installed, Python’s built-in http.server is one simple option, as described by MDN.
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- Open a terminal and change to the directory containing the files you want to serve.
- Start the server with
python -m http.server 8000. On systems where Python is invoked aspython3, usepython3 -m http.server 8000. - Leave the terminal process running, then open
http://localhost:8000in your browser. If the server reports a different port, use that port instead. - When finished, stop the server in the terminal, typically with
Ctrl+C.
This command serves files; it does not run server-side code for a framework or application that needs its own runtime. Use that project’s documented development command for those cases.
Why localhost is different from opening a file
Opening an HTML file directly creates a file:// URL, not an HTTP website. That distinction matters when the page uses server-side processing, asynchronous requests, or related files subject to browser origin restrictions. A page that appears to work as a file may behave differently when served over HTTP, and some file-based requests may fail. MDN discusses these issues in its local server guide and explanation of CORS errors for non-HTTP URLs.
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If a project depends on HTTP behavior, test it through a local server rather than treating file:// as an equivalent setup. That also gives you a more representative way to exercise browser requests and server-side behavior before deploying to a remote environment.
Localhost, HTTPS, and secure browser features
Modern browser secure-context guidance treats localhost and loopback origins as potentially trustworthy for certain secure-context checks. This can make some features that require a secure context available during local development. It does not mean that arbitrary HTTP websites are secure, nor does it guarantee identical behavior for every browser feature or context. Consult MDN’s secure-context guidance and verify behavior in the browsers you support.
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When a website tries to reach localhost
A different situation arises when a public website attempts to contact a service on your computer or local network. Those requests can potentially reach devices such as routers or printers, so browsers have introduced local-network access protections. Browser support and behavior vary; a page loading successfully at localhost does not establish that an unrelated HTTPS page can make requests to it.
MDN describes local network access controls and notes that the Request.targetAddressSpace API has limited availability and is experimental. Treat implementation details as browser-dependent and check compatibility in the target environment rather than relying on them as universal behavior.
Common localhost problems and fixes
- The browser says it cannot connect: confirm the development server is still running and use the host and port it reports. A stopped server or mismatched port means nothing is listening at that URL.
- The port is already in use: another process may be using the requested port. Stop that process if appropriate, or configure your development server to use another available port and open the updated URL.
- The page loads but requests fail: check whether you opened a
file://URL, whether the project expects an HTTP server, and whether it needs a framework-specific server. Browser origin restrictions can affect file-based requests. - Another device cannot open your localhost URL: on that device, localhost refers to the device itself. For network testing, configure the server to listen on a suitable network interface and use the host computer’s reachable local-network address, subject to firewall and network settings.
- A remote site cannot contact a local service: browser local-network protections may restrict the request. Check the browser’s permission behavior and relevant compatibility guidance; do not assume that localhost access from your own tab grants another site access.
Or skip the browser setup
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