Get Hostname from IP address/MAC address
Solution 1:
The command host IP should do it:
jed@jed-osx:~$ host 4.2.2.2
2.2.2.4.in-addr.arpa domain name pointer vnsc-bak.sys.gtei.net.
As Zoredache mentions in the comments, this is not going to work if DNS isn't set up properly. Of course, if that is the case, then you have very limited options. The only method that I can think of is to connect to the IP and ask it what its hostname is (e.g. if you can ssh to it, you can then perform the hostname command to see what it thinks its name is: ssh user@the-ip-address hostname).