Is it possible in windows cmd line to check all of the network addresses (with ping or similar) to see which ones are taken/ have active devices:
ie. something that does
All you are wanting to do is to see if computers are connected to the network and to gather their IP addresses. You can utilize angryIP scanner: http://angryip.org/ to see what IP addresses are in use on a particular subnet or groups of subnets.
I have found this tool very helpful when trying to see what IPs are being used that are not located inside of my DHCP.