They can get it quite accurate, but not completely accurate. The RIAA (Recording Industry Association of America) trials are an interesting example of how effective IP addresses can be tracked. A lot of people that were up for thousands of dollars in damages managed to avoid being successfully sued because their IP address wasn't specific enough to them.
I have a feeling that things will get even more difficult from a tracking perspective when IPv6 becomes standard. We're supposedly running out of IP addresses as is, so it's not as if there isn't a lot of clutter for people to sift through.
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