127.0.0.1 is a reserved IP address corresponding to the host computer. Known as the loopback address, 127.0.0.1 is used whenever a program needs to access a network service running on the same computer as itself. Although mainly used as a testing and development address, the loopback address can be used to access local services, such as webservers, that are usually only accessed over a network and have no local interface. Additionally, most modern operating systems that implement TCP/IP regard the name "localhost" as being equivalent to 127.0.0.1.
The address 127.0.0.1 is defined by the TCP/IP protocol as a reserved address that routes packets back to the host. Thus, no computer connected to the Internet, or any other TCP/IP compliant network, can identify itself as 127.0.0.1. The Internet Engineering Task Force's RFC 3330 defines 127.0.0.1 and 12 other special-use IPv4 addresses. Any public router or gateway that receives a packet destined for a special-use IP is required to drop it, without logging it's contents. Thus, if such a packet is accidentally forwarded outside of it's host, the packet will not accidentally arrive at another location that is willing to accept it and answer. This requirement helps provide network security, as even a machine that is configured to appear invisible on a network will likely answer packets addressed to it's loopback address. Additionally, some services may be unexpectedly activated by responding to such a stray packet.
A successful ping request to 127.0.0.1 will verify that a computer's network interface card, it's drivers, and the operating system's TCP/IP implementation are all functioning correctly. However, in addition to it's use as a troubleshooting and local access tool, 127.0.0.1 is often used in practical jokes and scams. One popular prank is to assign first-year computer science students to either crack or otherwise probe 127.0.0.1. Other variations include service technicians who 'prove' their network's speed and reliability by having customers ping the loopback address. Many anecdotes about the "World's Worse Hacker" performing a DDoS attack, or similar malicious feat, on 127.0.0.1 circulate the Internet.
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