utility that scans lan for client?

[John Almberg]

I've tried googling for this, but I guess I don't know the name of a  
utility such as this...

What I'm looking for is a utility that can scan a LAN for attached  
clients... i.e., computers that are attached to the LAN.

I have one box (an appliance that I have no access to), that is on  
the LAN but I don't know what IP address it's using. I'd like to  
complete my network map, and that is the one empty box on my chart.

Yes, I am obsessive :-)

Any help, much appreciated.

-- John