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In computer networking, localhost is a hostname that means this computer. It is used to access the network services that are running on the host via its loopback network interface. Using the loopback interface bypasses any local network interface hardware.
The local loopback mechanism is useful for testing software during development, independently of any networking configurations. For example, if a computer has been configured to provide a website, directing a locally running web browser to http://localhost may display its home page.
On most computer systems, localhost resolves to the IP address 127.0.0.1, which is the most commonly used IPv4 loopback address, and to the IPv6 loopback address::1.
The name localhost is also a reserved top-level domain name, set aside to avoid confusion with the definition as a hostname. The IETF standards restrict domain name registrars from assigning the name localhost in registration procedures, such as for second-level domains.
IPv4 network standards reserve the entire 127.0.0.0/8 address block for loopback purposes. That means any packet sent to one of those 16,777,214 addresses (127.0.0.1 through 127.255.255.254) is looped back. IPv6 has just a single address,::1.
The resolution of the name localhost into one or more IP addresses is configured by the following lines in the operating system’s hosts file:
127.0.0.1 localhost ::1 localhost
However, it may also be resolved with the Domain Name System (DNS). Localhost may be mapped to other IPv4 loopback addresses, and additional names may be assigned to any loopback address.
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. The standard sets forth a number of special considerations governing the use of the name in the Domain Name System:
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.
Applications may resolve the name to a loopback address themselves, or pass it to the local name resolver mechanisms.
When a name resolver receives an address (An or AAAA) query for localhost, it should return the appropriate loopback addresses and negative responses for any other requested record types. Queries for localhost should not be sent to caching name servers.
The releases of the MySQL database differentiate between the use of the hostname localhost and the use of the addresses 127.0.0.1 and::1. When using localhost as the destination in a client connector interface of an application, the MySQL application programming interface connects to the database using a Unix domain socket, while a TCP connection via the loopback address requires the direct use of the explicit address.
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