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The term 'loop-back' has the following meanings:
'1.' A method of performing transmission tests of access lines from the serving switching center, which method usually does not require the assistance of personnel at the served terminal.
'2.' A method of testing between stations (not necessarily adjacent) wherein two lines are used, with the testing being done at one station and the two lines interconnected at the distant station. Commonly called loop around when the interconnecting circuit is accessed by dialing.
'3.' A patch cable, applied manually or automatically, remotely or locally, that facilitates a loop-back test.
A 'loopback' is a communications channel with only one endpoint. Any message transmitted through such a channel is immediately received by the same channel.
A serial communications transceiver can use loopback for testing its functionality. For example, a device's ''transmit'' pin connected to its ''receive'' pin will result in the device receiving exactly what it transmits. Moving this looping connection to the remote end of a cable adds the cable to this test. Moving it to the far end of a modem link extends the test further. This is a common troubleshooting technique and is often combined with a specialized test device that sends specific patterns and counts any errors that come back (see Bit Error Rate Tester (BERT)). Some devices include built-in loopback capability.
Internet Protocol (IP) specifies a ''loopback network''. Under IPv4, this should be network 0 ("this network") and a host's local loopback address should be 0.0.0.0 ("this host on this network") but the historical misuse of this address (in particular, the use of host address 0 as a broadcast address) led to the technically questionable use of 127.0.0.1 as a "loopback address" instead. Because prior to CIDR, network 127 had a default netmask of 255.0.0.0 (a "Class A" network) this effectively resulted in the permanent loss of network 127.0.0.0/8, representing 1/256 of the total available IPv4 address space. Today most IPv4 implementations use 127.0.0.1 as the loopback address by default; a few do not even support the correct 0.0.0.0 at all.
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''. A loopback interface is a type of 'circuitless IP address' or 'virtual IP' address, as the IP address is not associated with any one particular interface (or circuit) on the host or router.
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 [1]. 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.
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"''. [2]
Some network equipment uses the term 'loopback' for a virtual interface used for management purposes. Unlike a proper loopback interface, this '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 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.
★ Loop (telecommunication)
★ Loop around
'1.' A method of performing transmission tests of access lines from the serving switching center, which method usually does not require the assistance of personnel at the served terminal.
'2.' A method of testing between stations (not necessarily adjacent) wherein two lines are used, with the testing being done at one station and the two lines interconnected at the distant station. Commonly called loop around when the interconnecting circuit is accessed by dialing.
'3.' A patch cable, applied manually or automatically, remotely or locally, that facilitates a loop-back test.
A 'loopback' is a communications channel with only one endpoint. Any message transmitted through such a channel is immediately received by the same channel.
| Contents |
| Serial Interface |
| Virtual Internet Protocol Network Interface |
| Network Equipment |
| See also |
Serial Interface
A serial communications transceiver can use loopback for testing its functionality. For example, a device's ''transmit'' pin connected to its ''receive'' pin will result in the device receiving exactly what it transmits. Moving this looping connection to the remote end of a cable adds the cable to this test. Moving it to the far end of a modem link extends the test further. This is a common troubleshooting technique and is often combined with a specialized test device that sends specific patterns and counts any errors that come back (see Bit Error Rate Tester (BERT)). Some devices include built-in loopback capability.
Virtual Internet Protocol Network Interface
Internet Protocol (IP) specifies a ''loopback network''. Under IPv4, this should be network 0 ("this network") and a host's local loopback address should be 0.0.0.0 ("this host on this network") but the historical misuse of this address (in particular, the use of host address 0 as a broadcast address) led to the technically questionable use of 127.0.0.1 as a "loopback address" instead. Because prior to CIDR, network 127 had a default netmask of 255.0.0.0 (a "Class A" network) this effectively resulted in the permanent loss of network 127.0.0.0/8, representing 1/256 of the total available IPv4 address space. Today most IPv4 implementations use 127.0.0.1 as the loopback address by default; a few do not even support the correct 0.0.0.0 at all.
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''. A loopback interface is a type of 'circuitless IP address' or 'virtual IP' address, as the IP address is not associated with any one particular interface (or circuit) on the host or router.
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 [1]. 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.
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"''. [2]
Network Equipment
Some network equipment uses the term 'loopback' for a virtual interface used for management purposes. Unlike a proper loopback interface, this '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 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.
See also
★ Loop (telecommunication)
★ Loop around
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