[ale] IP ?

[Jonathan]

I run NTOP on our LAN. This LAN is part of a WAN that is firewalled.  I
have 
no control over the firewall.  I noticed an IP address had downloaded
450 megs from somewhere on my LAN. I tried pinging the address and
couldn't reach it from my LAN ditto http and nslookup. 

I came home and logged on to my ISP. I could ping the ip but could not
ftp, 
telnet, or http to the address. Nslookup failed.  

I should also note that NTOP reported most of the traffic as UDP.

My question is how do I figure out what happened? Is there anyway of
determing 
what the IP is?

TIA