What the 127.0.0.1 IP Address Means

For performing tests, it is necessary for a PC software developer or user to know how IP addresses work. With a thorough understanding of IP addresses like 127.0.0.1, debugging and studying PCs becomes more straightforward.

Behind all the names on the urls of Internet websites are a series of numbers called IP (Internet Protocol) addresses. IP stands for Internet Protocol, and constitute the primary network layer for use on the Web.

The 127.0.0.1 IP

This particular IP address, also called a local host, is the basis for which loopback network connections are processed. Loopback means going back, and that is what 127.0.0.1 does, because it is the address of your machine. Using telnet, ftp, or try in any way to get to 127.0.0.1, you will be transported back to the host machine, yours. However, it is only the initial three numbers (127) that are needed; use any number combination with 127 and the result will be the same.

A frequent joke among programmers is to get the greenhorn to connect to this IP address. However, in reality the IP address has serious functions.

Uses and Purpose

There are several practical uses for this IP address. The most common example would for a software developer or network systems administrator to try out new applications or experiment with unique set ups. This can also be used by programmers who design software that talks to each other through the computer.

This IP address is also used for beta testing a host of web applications, from Java applets, Active X controls to web browsers.

It usually begins with the client sending out a message to server, which would be possible only if the 127.0.0.1 address is used. The result would be, if a web browser were to relay the request, a return to the host page of the site

The other popular usage of the local host IP address are gaming servers, which are interconnected to local hosts. By using the local hosts, the process and the flow of information becomes more streamlined and efficient.

Because it functions as a loopback, this IP address cannot be used in any network element or node.

Testing the Loopback Properties of 127.0.0.1

You do not need to be a computer programmer to test the loopback function of this address. You can go to the command prompt of your operating system, and at the c: type the following: ýping 127.0.0.1ý. After hitting the Enter key, you will get an answer like ýReply from 127.0.0.1ýý.

If you type in ýping localhostý instead of the numbers, the result would be the same, as localhost and 127.0.0.1 are the same.
You can also try it on telnet: assuming that the computer assigned to you has the name ýDaný you will get the following result:

# telnet 127.0.0.1
Trying 127.0.0.1...
Connected to dan
Escape character is '^]'

Currently IPv4 is the standard in use on the Internet today. Aside from 127.0.0.1, other reserved addresses are 10.0.0.0/8 (for private networks), 169.254.0.0/16 (for link local),

Others are 192.88.99.0/24 (relays from IPv6 to UPv4), 255.255.255 (for broadcast), 224.0.0.0/4 for Multicasts (former Class D network), 240.0.0.0/4 for Class E Network, 192.0.2.0/24 for Documentation and example code and 198.18.0.0/15 for Network benchmark tests; 172.16.0.0/12 is also used for private networks.

Peter Garant is writing articles about 127.0.0.1 localhost for a site about the 127.0.0.1 IP number in local networks.