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What is localhost?

James Gallagher - January 08, 2021


Localhost. What a peculiar domain name. There’s no “.com” or “.co” at the end of this one. Localhost stands alone as its own domain. As a developer or a techie, you’ve probably encountered localhost at some point. Perhaps you’ve seen it while developing a website or running an application on your computer.

What is localhost? In this guide, we discuss what localhost is, why it is important, and how it works.

What is Localhost?

Every device connected to the Internet has an IP address, which is used for identification. An IP address takes the form 1.1.1.1, with the “1s” replaced with any number between 1 and 255.

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Localhost is tied to a specific IP address: 127.0.0.1. This IP address refers to your computer, your local host. In other words, when your computer uses localhost, it is talking to itself. There are several standards protecting this IP address so that when you navigate to 127.0.0.1, you always connect to your computer.

Sometimes, localhost is prefixed with “http://”. This looks like http://localhost in your web browser. This URL allows you to enter a website hosted on your local machine.

What is Localhost Loopback?

Loopback refers to when your computer identifies you are accessing a local resource, rather than a resource on the outer web.

Computers use Internet Protocol (IP) addresses to communicate with each other. As we discussed earlier, every machine has its own unique IP. IP works with a protocol called Transmission Control Protocol (TCP) to connect to other devices. Maybe you’ve heard of TCP/IP in networking; that’s where it comes from.

All addresses between the range 127.0.0.1 and 127.255.255.255 are reserved for your local network by the Domain Name System (DNS). This is because there are several services that need to be hosted on your local network and should not be accessible to people outside. Your router is one example. You and your computers are the only ones who need to access that router.

The Internet Corporation for Assigned Names and Numbers (ICANN), who allocates domain names, has reserved these addresses, which means they are standard.

When you navigate to an IP address starting with 127, loopback addresses the query. Your computer connects to a resource on your local network, instead of on the web. It’s this mechanism that allows localhost to work. Without it, the keyword wouldn’t be private.

Where is Localhost Used?

We’ve determined that localhost is a special IP address on your computer. But, what is it used for?

Networking Testing

Network administrators use localhost to test their networks. Many parts of a network are insular, which means they can only be accessed within the network. Having reserved IP addresses makes it easy to test if a network is operational. It also allows administrators to run resources that can only be accessed by members of their network.

Application Testing

Web developers use this keyword during application development. For example, when building a website, they may not want to make it accessible to the public until it’s complete.

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Web developers can run web servers on their computers—those servers can only be accessed from that machine, and all connections stay within the developer’s system. This means that a developer can work on an application on their home computer in private. That application will only go public when it is pushed to a virtual server.

As a Synonym

Localhost is a synonym for 127.0.0.1, which is the IP for your computer on your local network. “localhost” is easier to say—and remember—than 127.0.0.1.

This synonym function works using a file called “hosts” which is on all Windows and Unix-based systems. On Linux operating systems, you’ll find the hosts file at /etc/hosts.

Assuming you haven’t changed the file yourself, you will see these records:

127.0.0.1 localhost

::1 localhost

These records tell your local computer that the term “localhost” should connect to 127.0.0.1 or ::1 (which both refer to your local computer).

Wrapping Up

Localhost is an essential part of networking. This concept allows network administrators and web developers to test their applications without connecting to the web. “Localhost” is easier to remember and say than 127.0.0.1; that’s why web developers use this term in their application. It’s just more convenient.

Now you can talk about the local host like a networking expert!

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James Gallagher

About the author: James Gallagher is a self-taught programmer and the technical content manager at Career Karma. He has experience in range of programming languages and extensive expertise in Python, HTML, CSS, and JavaScript. James has written hundreds of programming tutorials, and he frequently contributes to publications like Codecademy, Treehouse, Repl.it, Afrotech, and others.

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