127.0.0.1 and localhost explained

127.0.0.1 is the address every device uses for itself. Data sent to it never leaves the device.

The loopback address

The whole block from 127.0.0.0 to 127.255.255.255 is reserved for loopback: traffic that a device sends to itself. Nearly everyone uses just the first usable address, 127.0.0.1. The name localhost points to it on every system. In IPv6 the same job is done by the single address ::1.

When a program connects to 127.0.0.1, the data goes down into the system's network software and straight back up again. No network card, cable, Wi-Fi or router is involved. It works with the internet unplugged.

What it is used for

• Building and testing websites and apps. A developer runs a web server on their own computer and opens http://localhost:3000 or http://127.0.0.1:8080 in the browser. The number after the colon is the port, which selects one program among the many on the computer. See common port numbers.
• Programs talking to each other on one computer, such as a database and the app that uses it, without exposing either to the network.
• Local control pages. Some printer tools, sync apps and media servers show their settings as a page on localhost.
• Testing that the network software itself works:

ping 127.0.0.1

If this fails, the fault is in the computer's own network set-up, not in the cable or Wi-Fi.

"127.0.0.1 refused to connect"

This browser message means: your computer is fine, but no program is listening on that port right now. Usual reasons:

1. The local server or app is not running. Start it.
2. It is running on a different port. Check the number it printed when it started.
3. It is listening on the IPv6 address only, or the IPv4 one only. Try localhost, 127.0.0.1 and [::1] in turn.
4. A firewall or security program is blocking it.
5. You followed a link meant for someone else's computer. An address with 127.0.0.1 in it only ever works on the machine that runs the program; it can never be shared with another person.

Addresses that look similar

The difference between 127.0.0.1 and 0.0.0.0 matters for safety. A test server bound to 127.0.0.1 can be reached only from the same computer. Bound to 0.0.0.0, it can be reached by everyone on the same Wi-Fi, which you may not want in a cafe.

The hosts file

Every system has a small text file named hosts that is checked before DNS. It is where "localhost means 127.0.0.1" is written down. Pointing a site's name at 127.0.0.1 in this file is an old way to block that site on one computer: the browser then looks for it on the device itself and finds nothing. If a site you need suddenly fails on one computer only, an entry in this file left by some tool is worth checking.