Most ISP's don't give a permanent IP address to their customers, which means that any computer user can have a different IP address any time they get online. This is an economic issue. ISP's can't afford to give everyone a permanent IP address. Or we wouldn't want to pay the price. So in essence when Renderosity blocks an IP address, they're blocking a range of IP addresses which belong to a bunch of users in a given area. That's not a very good practice when you think about it. But then again that should be common knowledge to anyone who runs this kind of site or uses that kind of software.