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In computing, within the context of a TCP/IP network such as the Internet, localhost is a reserved name meaning this computer. It is used where one would otherwise specify the name or address of a computer in the network. For example, directing a web browser to http://localhost will display the home page of the web site (if any) being served from the computer running the browser. A common basic test of the TCP/IP protocol stack on a machine is to enter this command at a shell prompt:

ping localhost

localhost always translates to the loopback IP address 127.0.0.1 in IPv4, or ::1 in IPv6 (see below).

Being able to communicate with one's local machine as though it were a remote machine is useful for the purposes of testing, and also to contact normally remote services (such as game servers) that happen to reside on the local machine.

IETF document "Special-Use IPv4 Addresses" (RFC 3330) describes the 127.0.0.0/8 as a reserved IPv4 address block for loopback.

This address block will not be allocated or assigned to any organization or ISP. Any packet with an address of this address block '127.0.0.0/8' cannot appear outside of a host system. Within a host system, the loopback interface is normally assigned the address '127.0.0.1' with subnetwork mask '255.0.0.0'. This makes the routing table of the local system set with a routing entry of '127.0.0.0/8', so that packets destined to any address of this '127.0.0.0/8' would be routed internally within the local system.

On the other hand, only one IPv6 address ::1/128 (that is to say, the address with a one at its least significant bit and zero otherwise) is specified as loopback address in IPv6 addressing architecture specification of RFC 3513.

In RFC 3513, the specification says: "The loopback address must not be used as the source address in IPv6 packets that are sent outside of a single node. An IPv6 packet with a destination address of loopback must never be sent outside of a single node and must never be forwarded by an IPv6 router. A packet received on an interface with destination address of loopback must be dropped."

• An urban legend involving 127.0.0.1 is often circulated among the more technical computer related forums on the web. The story often involves a antigonist often referred to as “The World’s Worst Computer Hacker”. The story generally involves the Hacker hacking himself when tricked into using 127.0.0.1 as the IP address to hack into.^[1][2]

• In the same vein, 127.0.0.1 is often used as an IP address given to inexperienced computer users as a practical joke. Another story involves a user who, after finding that pinging 127.0.0.1 pings his computer, tells all of his associates that his IP is 127.0.0.1.

• ThinkGeek sells a T-shirt which reads "There's no place like 127.0.0.1"^[3]

1. ^ Elch. "M4tr1x-Trilogie", stophiphop.de, 2005-04-06. Retrieved on 2007-02-23. 
2. ^ Illiad (2001-05-23). Broken Mail Server. User Friendly.
3. ^ ThinkGeek, Tshirts > Generic Geek > There's no place like 127.0.0.1

• RFC 3330: "Special-Use IPv4 Addresses"
• RFC 3513: "Internet Protocol Version 6 (IPv6) Addressing Architecture"
• Your Loopback (Port 80)