What Is The Loopback Address?

Date First Published: 29th May 2022

Topic: Computer Networking

Subtopic: Network Identifiers

Article Type: Computer Terms & Definitions

Difficulty Level: 6/10

Learn more about what the loopback address in this article.

The loopback address, also known as the localhost address, is the reserved IP address that sends and routes data packets to its own computer system for testing. In other words, any traffic that a computer system sends on the loopback address is addressed to the same computer. It is not used for communicating with any other devices on a network. It is just used for accessing services on a local computer. Unless it is blocked by the pinging, this address by typing ‘ping 127.0.0.1’ in Command Prompt will always return a reply, as shown below.

Note:

On the internet, 127.0.0.1 is often jokingly used to refer to a home by the phrase: 'There is no place like 127.0.0.1', meaning that there is no place like home. This is because localhost and 127.0.0.1 are often referred to as the home device.

IPv4 and IPv6 Loopback Address

In IPv4, the most commonly used loopback address is 127.0.0.1. However, the address can range from 127.0.0.0 to 127.255.255.255. The number ‘127’ at the start of an IPv4 address is reserved for loopbacks, meaning that no networks or servers can have an IP address that starts with ‘127’. After entering any IP address that starts with that number, TCP/IP networks recognise that the local computer wants to be contacted. In IPv6, the loopback address is 0:0:0:0:0:0:0:1, which can be shortened to ::1.

Examples Of Loopback Use

The loopback address is commonly used when installing local web server software on a computer, such as Apache. The computer acts as a local web server and handles HTTP requests. In order to access the webpages, the loopback IPv4 address ‘127.0.0.1’ or the loopback domain name, ‘localhost’ is entered into a web browser with the location of the webpage. Overall, it is beneficial for developers as they can test websites without displaying them to the public.

Sometimes, the loopback address may be ended in a port number, such as ‘127.0.0.1:8080’. This is used when specifying a custom port number for a web server. Port 80 is the default number used for HTTP. Note that port 8080 is not reserved for alternative web servers.

The loopback address is also used by protocols, such as Open Shortest Path First (OSPF) to determine protocol-specific properties of the device or network. Stateless firewall filters can also be applied to the loopback address to filter outgoing and incoming traffic.