An IP address is very much like a listing in the telephone book. It's public information, you could ask for it.
If you have access to a router that has passed traffic from the machine in question, the arp tables would have that information. You would have to know the hostname to match it with the IP address in question, but there are ways that you could narrow down just one host.
I'd recommend reading more on networking and TCP/IP. You'll get some ideas.

Regards.

Edit : I am naturally assuming you are the admin of the network in question and have ownership of the IP/ machine you are looking to find out about.