Why can I ping a machines IP address and get a return but when I ping its host name I get a "destination unreachable" message?
The problem lies in the way that the hostname is resolved by your computer. If name resolution is not available within your network you need to add the actual IP address and hostname into your computers LMHosts lookup file. This is found at the following location:
Editing this file will allow you to associate an IP address with the hostname. The following should already be added and any subsequent additions take the same form.
This will remove the need to enter in a computers IP address to the agent options when testing remotely. If you are using a DHCP server to assign IP addresses dynamically then this will need changed any time a new IP is assigned to the machine in question.