Editing Loopback (section)

=={{Anchor|LOOPBACK-INTERFACE}}Virtual loopback interface==
{{redirect|Loopback address|reserved loopback hostname|localhost}}

Implementations of the [[Internet protocol suite]] include a [[virtual network interface]] through which network applications can communicate when executing on the same machine. It is implemented entirely within the operating system's networking software and passes no packets to any [[network interface controller]]. Any traffic that a computer program sends to a loopback IP address is simply and immediately passed back up the network software stack as if it had been received from another device. [[Unix-like]] systems usually name this loopback interface <code>lo</code> or <code>lo0</code>.

Various [[Internet Engineering Task Force]] (IETF) standards reserve the IPv4 address block {{IPaddr|127.0.0.0|8}}, in [[CIDR notation]] and the IPv6 address {{IPaddr|::1|128}} for this purpose. The most common IPv4 address used is {{IPaddr|127.0.0.1}}. Commonly these loopback addresses are mapped to the hostnames ''[[localhost]]'' or ''loopback''.

===MPLS===
An exceptional (non-loopback) use of {{IPaddr|127.0.0.0|8}} network addresses is in [[Multiprotocol Label Switching]] (MPLS) traceroute error detection, in which their property of not being routable provides a convenient means to avoid delivery of faulty packets to end users.

===Martian packets===
Any IP [[datagram]] with a source or destination address set to a loopback address must not appear outside of a computing system, or be routed by any routing device. Packets received on an interface with a loopback destination address must be dropped. Such packets are sometimes referred to as [[Martian packet]]s.<ref>{{cite web|author=Raymond, Eric S.|title=The Jargon File|url=http://www.catb.org/~esr/jargon/html/M/martian.html|access-date=2004-06-23|archive-date=2020-11-06|archive-url=https://web.archive.org/web/20201106220734/http://www.catb.org/~esr/jargon/html/M/martian.html|url-status=live}}</ref> As with other bogus packets, they may be malicious and any problems they might cause can be avoided by applying [[bogon filtering]].

===Management interface===
Some computer network equipment use the term "loopback" for a virtual interface used for management purposes.  Unlike a proper loopback interface, this type of loopback device is not used to talk with itself.

Such an interface is assigned an address that can be accessed from management equipment over a network but is not assigned to any of the physical interfaces on the device.  Such a loopback device is also used for management datagrams, such as alarms, originating from the equipment.  The property that makes this virtual interface special is that applications that use it will send or receive traffic using the address assigned to the virtual interface as opposed to the address on the physical interface through which the traffic passes.

Loopback interfaces of this sort are often used in the operation of [[routing protocol]]s, because they have the useful property that, unlike real physical interfaces, they will not go down when a physical port fails.