600 kbit/sec is 0.6% of 100mbit.
When downloading from the internet you will never achieve more then 0.6% utilization.
(Unless you meant 600 kilobytes per second .. which is about 5 mbit .... then you would max out at about 5% when downloading from the internet)
Now if you were copying files from one computer on your network to another computer on your network, you should be able to get a LOT more speed, typically about 40 to 60 mbits/sec (maybe 80 to 90 mbit if your system has awesome I/O capabillities and an insanely fast CPU)