A good tips on how to make an inverntory of a large network is found on isc.sans.org: Link
Using the free tool p0f connected to network span ports at central points, should give you quite good insight in the network traffic, without needing credentials.
P0f is a tool that utilizes an array of sophisticated, purely passive traffic fingerprinting mechanisms to identify the players behind any incidental TCP/IP communications (often as little as a single normal SYN) without interfering in any way.
A challenge would be to visualize the traffic. It would be interesting testing this a s a source for the HP uCMDB.