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The beauty of Internet is one side has only take care of what happens in their area of responsibility and traffic is correctly routed inside it and to outside from it. And this is done by combining lot of individual addresses in blocks(CIDR). Also routing can't be considered constant even during connection, it can change due load-distribution.
ISP can record to which subscriber particular IP was assigned at certain time(not overly intensive operation, they don't change very often afterall). And also certain areas tend to use same IP-ranges for routing purposes. This information can be attained, but due to open WLANs the real responsible party might be impossible to find.
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