Ok, I've got a very basic question, just need to know what command I'm looking for.

I've got a new network monitoring program and I'm trying to figure out which IP addresses it shows belongs to which machines in this building. I have them all identified except for a few, and those few I have no idea for the life of me who the addresses belong to.

So what I want to do is use the IP address to find the host name. I know this is a basic thing, so I just need to remember what the command to use is. I'm used to pinging the host name to find the IP, so I'm just trying to figure out how to do vice-versa.