Localhost

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In computing the term localhost refers to the location of the currently used system. It is a loopback device which is assigned the IP address 127.0.0.1 in IPv4, or ::1 in IPv6, which can be used by TCP/IP applications to talk to themselves if this is needed. If you are running a server, localhost will most likely open a page. [1]

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 services (such as game servers) located on one's own machine that are expected to be remote.

Related Specification of IETF

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."

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

Humor

A typical prank played on newbies and script kiddies is giving them the IP address 127.0.0.1, claiming it is a server with lots of warez or pornography, or when challenging the boastful script kiddies to hack some computer. The script kiddies thus frequently manage to get themselves dropped off the net when they use DoS tool on themselves (For example, [2]). Some manage to destroy their own systems (A chat transcript exists of one such case [3]) A variant of the joke involves a lot of DNS records, typically in form "warez.example.net", that all resolve to 127.0.0.1 ([4]).