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A loopback is a communications channel with only one endpoint. Any message transmitted through such a channel is immediately received by the selfsame channel.
The Internet protocol (IP) specifies a loopback network. Under IPv4, this has the CIDR address of 127.0.0.0/8. Most IP implementations support a loopback interface, which represents the loopback facility. Any traffic that a computer program sends on the loopback network is addressed to the same computer. The most commonly used IP address on the loopback network is 127.0.0.1 for IPv4 and ::1 for IPv6. The standard domain name for this address is localhost.
On Unix-like systems, the loopback interface is commonly called 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 at the URL http://127.0.0.1/ will access that computer's own Web site. This can be done without the computer being connected to any network -- so it is useful for testing services without exposing them to remote network access. Likewise, to ping the loopback interface is a basic test that one's IP stack is working properly.
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. Many users are aware that 127.0.0.1 stands for their own computer, but not that (e.g.) 127.42.69.93 does as well.
Packets sent on a "real" IP network with a source address on the loopback interface can cause a number of problems for older or buggy network software. Such packets are known as "martian packets". [1] (http://www.catb.org/~esr/jargon/html/M/martian.html)
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