Applications may resolve the name to a loopback address themselves, or pass it to the local name resolver mechanisms.An IPv4 or IPv6 address query for the name localhost must always resolve to the respective loopback address, which is specified in a separate standard.The standard sets forth a number of special considerations governing the use of the name in the Domain Name System: The name localhost is reserved for loopback purposes by RFC 6761 ( Special-Use Domain Names), which achieved the Proposed Standard maturity level in February 2013. IETF standards prohibit domain name registrars from assigning the name localhost. In the Domain Name System, the name localhost is reserved as a top-level domain name, originally set aside to avoid confusion with the hostname used for loopback purposes. The mapping of localhost to addresses other than the designated loopback address range in the hosts file or in DNS is not guaranteed to have the desired effect, as applications may map the name internally. In addition to the mapping of localhost to the loopback addresses ( 127.0.0.1 and ::1), localhost may also be mapped to other IPv4 (loopback) addresses and it is also possible to assign other, or additional, names to any loopback address. The name may also be resolved by Domain Name System (DNS) servers, but queries for this name should be resolved locally, and should not be forwarded to remote name servers. The resolution of the name localhost to one or more IP addresses is normally configured by the following lines in the operating system's hosts file: The IPv6 standard assigns only a single address for loopback: ::1. However, they can be used to set up multiple server applications on the host, all listening on the same port number. The address 127.0.0.1 is the standard address for IPv4 loopback traffic the rest are not supported by all operating systems. That means any packet sent to any of those addresses is looped back. IPv4 network standards reserve the entire address block 127.0.0.0/8 (more than 16 million addresses) for loopback purposes. The name localhost normally resolves to the IPv4 loopback address 127.0.0.1, and to the IPv6 loopback address ::1. For example, a locally installed website may be accessed from a Web browser by the URL to display its home page. The local loopback mechanism may be used to run a network service on a host without requiring a physical network interface, or without making the service accessible from the networks the computer may be connected to. Using the loopback interface bypasses any local network interface hardware. It is used to access the network services that are running on the host via the loopback network interface. In computer networking, localhost is a hostname that refers to the current computer used to access it.
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