A successful ping test tells you that the remote system is reachable. This means that the interface configuration of both the local and the remote system must be correct, the routing configuration of both systems must be correct, and the network hardware must be operational end to end. A successful test eliminates a lot of potential problems, allowing you to concentrate your testing.
For example, assume that a user reported that she was unable to use ftp to connect to dog.example.org. You could run a ping test. A successful ping test is shown in Listing 13.9.
Listing 13.9: A Successful ping Test
$ ping dog.example.org
PING dog.example.org (18.104.22.168): 56 data bytes 64 bytes from 22.214.171.124: icmp_seq=0 ttl=32 time=1.0 ms 64 bytes from 126.96.36.199: icmp_seq=1 ttl=32 time=0.7 ms 64 bytes from 188.8.131.52: icmp_seq=2 ttl=32 time=0.7 ms 64 bytes from 184.108.40.206: icmp_seq=3 ttl=32 time=0.7 ms AC
4 packets transmitted, 4 packets received, 0% packet loss round-trip min/avg/max = 0.7/0.7/1.0 ms
This test shows that the connection to dog.example.org is running perfectly. The summary line says that every packet transmitted was successfully echoed back, and none was lost. The individual lines report information about each packet. The ttl field prints the TCP Time-To-Live value. It is not significant here, but we will see later how the traceroute command makes use of this field. The sequence numbers (icmp_seq) tell you that every packet was received in sequence, and the round-trip times tell you that you have a fast connection to the remote site. A high packet loss, packets arriving out of sequence, or high round-trip times can indicate a congested network or a bad connection. But none of those things is happening in this example. Clearly, the user's problem lies elsewhere.
After verifying that the user's report about not being able to log in with ftp is true, check with the remote system administrator to see if they allow ftp access and are actually running the ftp daemon. Sometimes, services are blocked at the server or at the firewall for security reasons, and the user does not know it.
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