When you type "localhost" in your browser or use it in any networking context on your computer, it is resolved to the IP address 127.0.0.1.

Resolving localhost happens locally on your machine. Every operating system has a file called the hosts file, which is used to map hostnames to IP addresses.

• IPv4: localhost is typically mapped to the IPv4 address 127.0.0.1.
• IPv6: On systems that support IPv6, localhost is also mapped to the IPv6 address ::1.

This dual mapping ensures that localhost can be resolved to both IPv4 and IPv6 addresses, depending on the network protocol being used.

On a Unix-like system like Linux and MacOS, you can find the hosts file at /etc/hosts, and it usually contains these lines:

127.0.0.1   localhost
::1             localhost

Loopback Interface#

The loopback interface is a virtual network interface that your operating system uses to communicate with itself. It is identified by the IP address 127.0.0.1 for IPv4 and ::1 for IPv6. These addresses are called loopback addresses.

Any data sent to these addresses will be sent back to your own computer, thus making localhost refer to your own machine.

When you access localhost, your operating system’s networking stack interprets this as a request to communicate with the loopback interface. This allows developers to test applications on their local machine without requiring external network access.

Further Reading#

• Understanding the Difference Between localhost and 127.0.0.1 IP Addresses
• Localhost vs. 127.0.0.1