Web & Networking › Networking Fundamentals

localhost / 127.0.0.1

The address that always means "this machine".

Also known as: 127.0.0.1, loopback, local host

localhost is a special name that always means “this machine”. Connecting to it never leaves your computer. It uses the loopback address 127.0.0.1 (and ::1 in IPv6).

python -m http.server 8000
# now visit http://localhost:8000 in your browser
curl http://127.0.0.1:8000

The server and the client are both on your machine, so no network is involved.

Why it matters for development

• Run and test your app locally, with a dev server on localhost:3000, a database on localhost:5432, and so on.
• Safe by default. A service bound to 127.0.0.1 accepts connections only from the same machine, so others on the network can’t reach it.
• Fast, with no real network.

Binding addresses

The address a server listens on decides who can connect:

A common problem: a server works on your laptop but not from your phone, because it’s bound to localhost. Bind to 0.0.0.0 for LAN testing (and be careful about what you expose). In Docker, a server inside a container must listen on 0.0.0.0 for published ports to work (port mapping).

Gotchas

• localhost inside a container means the container itself, not your host machine. To reach the host, use the platform’s special name, or a Compose service name.
• localhost on a server means that server, not your laptop.
• IPv4 vs IPv6: localhost may resolve to ::1, while the server listens only on 127.0.0.1. If one works and the other doesn’t, check both.
• Browsers treat http://localhost as a secure context for many features, so you often don’t need HTTPS locally, but cookies and CORS still apply (CORS).
• The name comes from the hosts file, not the internet (DNS).