Loopback address is a special IP number (127.0.0.1) that is designated for the software loopback interface of a machine.
Loopback address, based upon RFC 5735: Special Use IPv4 Addresses, is a special IP number (eg: 127.0.0.1) that is designated for the software loopback interface of a machine. The loopback interface has no hardware associated with it, and it is not physically connected to a network.
Within IPV6 loopback looks like 0:0:0:0:0:0:0:1, although in IPV6, the longest contiguous block of zeros may be replaced with :: thus resulting in the condensed loopback address of ::1, based upon RFC 4291: IP Version 6 Addressing Architecture.
Looback address is a special range of addresses in the range 127.0.0.0 to 127.255.255.255. The data sent by a host to a 127.x.x.x loopback address are not passed down to the data link layer for transmission. Instead, they “loop back” to the source device.
The purpose of the loopback range is testing of the TCP/IP protocol implementation on a host. Since the lower layers are short-circuited, sending to a loopback address allows the higher layers (IP and above) to be effectively tested without the chance of problems at the lower layers manifesting themselves. 127.0.0.1 is the address most commonly used for testing purposes.
Sources:
- Internet History - What is an IP (Internet Protocol) address? (paragraph 1)
- The TCP/IP Guide - IP Reserved, Loopback and Private Addresses (paragraphs 2, 3)
