Localhost

Encyclopedia : L : LO : LOC : Localhost

This article is about the loopback device IP address. For information on the file system computer program, see localhost (computer program).

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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.

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."

Related humor

The slogan There is no place like 127.0.0.1 is sometimes seen on t-shirts from the OSTG initiative Thinkgeek and others. It plays on nerdy humor since 127.0.0.1 is here synonymous with home, by assuming that the loopback address refers to the home computer, reflecting on the saying that there is no place like home. This pun has various deviations including localhost and ::1 instead, the latter somehow also promoting IPv6 use.

External links

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

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