[spiffytech]

Just IP comparison only works if the user's IP is constant, which rules out many phones, tablets, and laptops that frequent coffee shops and restaurant wifi. It also doesn't allow you to tell when IP $a and IP $b are really the same person at home and work. Further, IP comparison doesn't let you distinguish between multiple users coming from the same home or office network, or from a proxy.

Commercial implementations of this sort of tracking include the user's IP in their dataset, but track a number of other datapoints so they can tell if e.g. everything but a user's IP matches an entry in their database, it's probably the same person connecting from a different location.