What is localhost and why is 127.0.0.1 used?

Published on 2026-10-17 by frfrand

Entendé localhost, 127.0.0.1, puertos locales y por qué una aplicación puede funcionar en tu PC pero no ser accesible desde Internet.

You are programming a website and the browser shows that it works at http://localhost:3000. You send that address to another person and they see absolutely nothing. Did the application break? No. Localhost refers to the same computer from which you make the request.

This confusion is so common that it is worth understanding well once. After that, many things about networks start to make sense.

What is localhost, exactly?

localhost is a name that normally points to a special address called loopback: 127.0.0.1 in IPv4 or ::1 in IPv6. That address does not look for another computer on the Internet. It tells the system: «I want to talk to this same machine».

It is like calling the internal phone of your own desk. You do not need to leave the building for the call to go through.

And the number after the colon?

In localhost:3000, the 3000 is the port. The address indicates which machine you want to reach and the port helps identify which service should receive the connection.

You can have an application running on port 3000, an API on port 8000, and a database listening on another. It is not advisable to think of a port as a physical door that is always open: if no program is listening and there are no rules redirecting traffic, no service will respond.

Why does it work on my PC and not on another machine?

Many development servers are configured to listen only on the loopback interface. This allows you to test an application without making it accessible to the entire network.

When you change the configuration to listen on 0.0.0.0, you tell a program to accept connections on all available IPv4 interfaces, but that does not mean the whole world can enter. The network, the firewall, a router, the provider's rules, and the address used still influence.

A simple way to distinguish it: - 127.0.0.1: communication with the same machine. - 192.168.x.x: a common example of private network addresses. - A public IP: an address that can be reachable from the Internet, according to routing and applied rules.

A real case: testing an API

Your frontend runs on localhost:3000 and your backend on localhost:8000. Both work on your notebook and talk to each other.

Now you upload the frontend to a platform and forget to change the backend URL. The visitor's application tries to connect to their own localhost:8000, not yours. That is why it fails even though the backend continues to work perfectly on your PC.

It is a small mistake, but it explains at once why local development and production are not the same.

Is it safe to use localhost?

It can reduce the exposure surface, but it does not replace application security. A local service can still be reached by processes on the same machine and, if there is a proxy or tunnel configured, it can end up accessible from elsewhere.

Do not expose admin panels, databases, or testing tools just because «it is more convenient». First decide who needs to enter and from where.

What happens in ArduMaker?

On a VPS managed from ArduMaker, an application can listen on localhost and receive visits through Nginx or a properly configured proxy. That way the public sees the website, but does not need direct access to the application's internal port.

In one sentence: localhost always points to the same machine from which it is used. Understanding this avoids many network mysteries and quite a few hours searching for errors that actually do not exist.