Retrieve Remote Machine IP

I have a network of considerable big size. The machines are all windows based. The server is linux. My requirement is to get the IPs of all the machines that come in contact with my network via any means i.e. chat, mail etc. To simplify the requirement I explain the scenario as below:

I want to get the remote machine’s ip that comes in contact with any user on any machine in my network. The contact may be thru chat. Actually thats primary.

So what I wanna know is how is it possible if at all it is?

Plz. provide your valuable inputs in this regard.