When an application program cannot successfully connect to the destination
host, is it a lower layer problem or a higher layer problem?

For
example, assume the problem is the inability to connect to a remote FTP server.
To determine whether this is an application layer problem and not a lower layer
problem, the first step is to verify Layer 3 connectivity. If successful, Layer
3 and lower can be eliminated as the source of the problem.
To
troubleshoot use the following steps:
- Ping the default gateway. If successful, Layer 1 and Layer 2 services are
functioning properly.
- Verify end-to-end connectivity. Use an extended ping if
attempting the ping from a Cisco router.
If these pings are successful, then Layer 1 through Layer 3 can be
eliminated. Since they are functioning properly, the issue must exist at a
higher layer.