# Localhost

URL: https://softwaredictionary.org/terms/localhost
Category: Networking
Last updated: 2026-10-03
Pronunciation: LOH-kul-hohst

In short: Localhost is the hostname for the computer you are using; it resolves to the loopback address 127.0.0.1 (::1 in IPv6), so its traffic stays on the machine.

## What is localhost?

Traffic sent to localhost goes into the operating system's network stack and comes straight back out through a virtual loopback interface; it never reaches the network card or the internet. In IPv4 the whole `127.0.0.0/8` range is reserved for loopback, with `127.0.0.1` the standard address, and in IPv6 the loopback address is `::1`.

Developers use it constantly. A dev server started with `npm run dev` is opened at `http://localhost:3000`, a local database listens on `localhost:5432`, and tools talk to each other through ports on the same machine. The port number tells the operating system which program should receive the traffic.

Browsers treat localhost as a secure context even over plain HTTP, so features that normally require HTTPS, such as service workers and the clipboard API, work during development. Many tools also distinguish `127.0.0.1` from `0.0.0.0`: a server bound to `127.0.0.1` accepts only local connections, while one bound to `0.0.0.0` listens on every network interface.

A common misconception is that localhost inside a Docker container means your computer. Each container has its own network namespace, so localhost there is the container itself; to reach a service on the host, use the address Docker provides, such as `host.docker.internal`, or put both services on the same Docker network.

## Key takeaways

- Localhost is the name for your own machine, the loopback address.
- It resolves to 127.0.0.1 in IPv4 and ::1 in IPv6.
- Loopback traffic never leaves the computer.
- Ports separate programs, as in localhost:3000 for a dev server.
- Inside a container, localhost is the container, not the host.

## Example: Binding a server to localhost or to every interface (Node.js)

```javascript
import http from "node:http";

const handler = (req, res) => res.end("Hello from this machine\n");

// Only reachable from this computer
http.createServer(handler).listen(3000, "127.0.0.1");

// Reachable from other devices on the network too (e.g. a phone on the same Wi-Fi)
http.createServer(handler).listen(3001, "0.0.0.0");

// curl http://localhost:3000   → works
// curl http://<your-LAN-IP>:3000 → refused; :3001 works
```

## Frequently asked questions

**What is the difference between localhost and 127.0.0.1?**

Localhost is a name and 127.0.0.1 is an address. The name usually resolves to 127.0.0.1, and on many systems also to ::1, the IPv6 loopback, which is why a server listening only on IPv4 can occasionally behave differently.

**Can other people access my localhost?**

No. Loopback traffic stays inside your machine. To let others reach a local server, it must listen on a network interface and be reachable through your network, or be exposed through a tunneling tool.

**What does localhost:3000 mean?**

Port 3000 on your own machine. Development servers commonly use ports such as 3000, 5173 or 8080, and the port tells the system which program should handle the request.

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Software Dictionary: https://softwaredictionary.org/ · https://softwaredictionary.org/llms.txt