What is Localhost or 127.0.0.1?

If you are just getting started with web development, web hosting or any form of network configuration, learning about the concept of localhost is a fundamental part.

In this tutorial, we will learn what the term localhost means, how it entails to the functionality of your devices, and some major uses of it.

What is the term Localhost?

The term localhost host is an alias referring to the network address 127.0.0.1 which is the default address for your loopback network interface or loopback address.

The loopback mechanism is a powerful networking feature that allows applications to route the traffic from the local machine back to itself.

According to the Internet Protocol specification, the IP address range between 127.0.0.1 to 127.255.255.255 is reserved for loopback address.

Therefore, although the most commonly used format is 127.0.0.1, the term localhost refers to all the IP ranges denoting the loopback address.

Remember, sending traffic to any address in the 127.0.0.0/8 range does not send traffic out of your machine. Instead, it’s rerouted internally (looped back) by the IP stack within your own operating system.

How the Loopback Address Works?

The following process depicts how the loopback address works in simple terms:

1. When network software on your machine sends a packet to the loopback IP address (typically 127.0.0.1 in IPv4 or ::1 in IPv6), the underlying network stack.
2. Instead of sending this packet out to the physical network, the network stack directly routes the packet back to the receiving part of the network stack on the same machine.
3. The network software receiving the packet will then process it as if it had arrived from an external network. The entire process completely bypasses any physical network hardware.
4. Since the packet never leaves your machine and doesn’t need to traverse any physical network hardware, the latency is extremely low and the transmission is reliable. It’s not affected by any network problems.

Pinging Localhost

To test whether your local machine is responing, you can perform a basic ping to the loopback address using the ping command as shown:

ping localhost

Or

ping 127.0.0.1

For IPv6, use the address:

ping ::1

Unless something is wrong with the networking stack of your operating system, pinging the loopback address will always be alive (unless its on a custom port).

Uses of Localhost

The following are some common uses of the loopback address in your system.

1. Testing Network Software - The loopback address is a valuable tool for us devs as it allows us to test new applications on the local machine before deployment. For example, you can use it to build a fully functional webserver to test your web apps before taking them to production.
2. Network Troubleshooting - The loopback address can also help diagnose network connectivity problems. If a user can reach services at the loopback address but not on the external network, the issue lies somewhere beyond the local machine.
3. Running Network Services on the Local Machine - Sometimes, you may want to run a network service (like a database server) that’s only accessible from your own machine, for security reasons or otherwise. By binding the service to the loopback address, you ensure that it’s not reachable from any other machine on the network.

And there you have it.

Conclusion

In this tutorial, we explored the concept of localhost, what it means, and how the loopback mechanism works in your system. We also discussed some possible uses of the loopback address and more.