Examples of applications include the following:
Correspondingly, the Internet Protocol (IP) specifies a loopback network. In IPv4 this is the network with the CIDR prefix 127/8 ("this network", RFC 3330). The most commonly used IP address on the loopback device is 127.0.0.1 for IPv4, although any address in the range 127.0.0.0 to 127.255.255.255 is mapped to it. IPv6 designates only a single address for this function, 0:0:0:0:0:0:0:1 (also written as ::1), having the ::1/128 prefix (RFC 3513). The standard, officially reserved, domain name for these addresses is localhost (RFC 2606).
On Unix-like systems, the loopback interface usually has the device name lo or lo0.
A loopback interface has several uses. It may be used by network client software on a computer to communicate with server software on the same computer, viz., on a computer running a web server, pointing a web browser to the URLs
http://localhost/ will access that computer's own web site. This works without any actual network connection–so it is useful for testing services without exposing them to security risks from remote network access. Likewise, pinging the loopback interface is a basic test of the functionality of the IP stack in the operating system.
Packets sent in an IP network with a source address belonging to the loopback interface can cause a number of problems for older or buggy network software. Such packets are known as "martian packets" The Internet Protocol specification dictates that such packets must not be transmitted outside of a host, and must be dropped if received on a network interface.
Loopback addresses are the subject of a reasonably common Internet prank: directing an inexperienced user to connect to (or attack) his own loopback interface as if it were a remote server Note, however, that contrary to popular belief, a computer will normally not crash if it "flood-pings" the loopback address. The only effect is a busy processor.
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 real interfaces on the device. This loopback address 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.
A hardware loop is a simple device that hardwires the receive channel to the transmit channel. In the case of a network termination connector such as X.21, this is typically done by simply connecting the pins together in the connector. Media such as optical fiber or coaxial cable, which have separate transmit and receive connectors, can simply be looped together with a single strand of the appropriate medium.
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