In addition to using SNMP monitoring tools for particular pieces of gear, you can also use third-party monitoring tools that actually check for the real application-level availability of any service or server that you choose. These tools generally
Get a list of IP numbers and services (port numbers) that the user wants to monitor.
Try to poll the services (check them using the appropriate protocol) every so often.
Request content as if an actual user were requesting it (and therefore can evaluate how an actual user perceives this service).
Evaluate the response: Did the services respond at all? Did they respond correctly? How fast did they respond?
Sound the alarm (via pager, email, and so on) if something doesn’t respond correctly. ...