I have a user who's machine is not pingable on our network, using computer hostname or associated IP. However, in troubleshooting this, I see that the IP the machine actually has does not resolve ...

Similarly, a company may map different IP addresses (one address per server, say) to a single hostname.

Like sonar, it returns a reply when it hits its target. Given an IP address, however, you can also use the Ping utility to return the TCP/IP host name of the Windows computer you’re tracing.