unable to telnet CCNA Test 1 Question

[pod7]

You are unable to telnet to a router at address 203.125.12.1 from a workstation with the IP address 203.125.12.23. You suspect that there is a problem with your protocol stack. Which of the following actions is most likely to confirm your diagnosis?

I do not understand why the answer is: ping 127.0.0.1

[Dione]

read page 98 in our softback book ip troubleshooting

[Overlord]

Another way of saying that the proposed cause of the problem is the protocol stack is to say that the TCP/IP software of the operating system is not working.

If you want to test if the TCP/IP software in the operating system is working, a software loopback test is the best way. Sure, a PING to another PC on the network does the job, but it can also fail if the other PC is down or inacessible. The software loopback does just the job of testing the TCP/IP software and nothing more.

Example - If a Windows update for TCP/IP and your network card driver fails, you would not be able to PING to IP address 127.0.0.1. If a PING to IP address 127.0.0.1 is successful, then you know the software on your computer is at least good. Other applications and actual network infrastrucutre factors remain.

Challenge - Use any IP address besides 127.0.0.1 in the 127.x.x.x subnet and see if there are any changes in the results. Then calculate how many IP addresses are in a class A subnet and ponder the allocation and use of this IP address range.

[muddyfox1009]

For troubleshooting, I followed what Todd lammle's instructions: with the ping test I start from local loopback, then the network address, then the gateway, and then the target machine. Any failure or correcponding failure would indicate where in the network the troubleshooting should start.