Information Technology Reference
In-Depth Information
Using a Command-Line Interface
The following command confirms that the TCP/IP loopback interface is responding correctly:
> ping loopback
How It Works
In TCP/IP networking, the loopback address is a special IP address that is reserved for diag-
nostic testing. When you ping 127.0.0.1 , you are essentially saying to your Windows Server
2003 computer, “Verify that my network interface card is functioning, that TCP/IP is installed
correctly, and that I've been assigned a valid IP address.”
It's important to note, however, that there is a difference between a valid IP address and a
correct one. You can configure a computer with an IP address that should not be in use on your
network—one that is in the incorrect subnet, for example—but as long as it is a valid IP address,
you will still be able to ping the loopback address on the local computer. For this reason, pinging
the loopback IP address is only the first step in troubleshooting connectivity issues; successfully
pinging the loopback address eliminates corruption of the TCP/IP stack or a hardware failure
on the local NIC as a cause of network failure. If you can successfully ping the loopback address
but are still having communications issues, you'll know that the problem lies elsewhere in your
computer's network configuration, or the configuration of the remote host to which you are
trying to connect.
While it is usually easiest to issue the ping command from the command line, you may
want to perform this task through a GUI as well. The Extended Ping (eping) utility is a free
download available from http://www.itoolpad.com/products/eping/ .
See Also
￿
iToolPad: Extended Ping utility ( http://www.itoolpad.com/products/eping/ ).
Wikipedia: “Loopback” ( http://en.wikipedia.org/wiki/Loopback ). This article
discusses the loopback address.
￿
Microsoft TechNet: “IP Addressing” ( http://www.microsoft.com/technet/itsolutions/
network/evaluate/technol/tcpipfund/tcpipfund_ch03.mspx ).
￿
9-3. Verifying the Path to a Remote Host
Problem
You want to verify that there is a valid network connection to a remote host, either on the same
subnet or a remote network.
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