For example, a locally installed website may be accessed from a Web browser by the URL http://localhost to display its home page.
However, they can be used to set up multiple server applications on the host, all listening on the same port number.
[1] In 1981, the block 127.0.0.0/8 got a 'reserved' status,[5] as not to assign it as a general purpose class A IP network.
Such packets are never passed to any network interface controller (NIC) or hardware device driver and must not appear outside of a computing system, or be routed by any router.
This permits software testing and local services, even in the absence of any hardware network interfaces.
Packets received on a non-loopback interface with a loopback source or destination address must be dropped.
One notable exception to the use of the 127.0.0.0/8 addresses is their use 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.