ARCHIVED: What is a loopback address?

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A loopback address is a special IP address, 127.0.0.1, reserved by InterNIC for use in testing network cards. This IP address corresponds to the software loopback interface of the network card, which does not have hardware associated with it, and does not require a physical connection to a network. The loopback address allows for a reliable method of testing the functionality of an Ethernet card and its drivers and software without a physical network. It also allows information technology professionals to test IP software without worrying about broken or corrupted drivers or hardware.

To test a network card using the loopback address, you can use the TCP/IP utility Ping. The best way to do this is with the Ping utility that comes with most operating systems. This is a simple command-line utility that will try to communicate to an IP address.

To run Ping, first follow the appropriate instructions below for your operating system:

  • Windows XP: Click Start and select Programs or All Programs, then Accessories, then Command Prompt.
  • Windows 2000: Click Start and select Programs, then Accessories, then Command Prompt.
  • Windows NT 4.0: Click Start and select Command Prompt.
  • Windows 95, 98, and Me: Click Start and select Programs, then MS-DOS Prompt.
  • Unix and Linux: Open a shell prompt.
  • Mac OS X: Open the Terminal.
  • Mac OS: Ping wasn't available as part of the OS in Mac OS 9 and earlier. For information on three utilities you can use instead, see ARCHIVED: In Mac OS, how can I check to see if a computer is on the network?

Once you are at a command prompt, enter the following:

  ping 127.0.0.1

If the command is successful, the Ping utility will return results similar to the following. The exact information returned will vary depending on your operating system:

  Pinging 127.0.0.1 with 32 bytes of data:

  Reply from 127.0.0.1: bytes=32 time<10ms TTL=128
  Reply from 127.0.0.1: bytes=32 time<10ms TTL=128
  Reply from 127.0.0.1: bytes=32 time<10ms TTL=128
  Reply from 127.0.0.1: bytes=32 time<10ms TTL=128

  Ping statistics for 127.0.0.1:
      Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
  Approximate round trip times in milli-seconds:
      Minimum = 0ms, Maximum =  0ms, Average =  0ms

This indicates that the network card and drivers are functioning properly. If the Ping utility is not able to get a return on the network card, this may indicate either a driver problem, or a physical problem with the card.

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Last modified on 2018-01-18 12:50:57.