If it weren't effectively property there wouldn't be firms listing large blocks of v4 space as an asset while going out of business, and there wouldn't be brokers specializing in acquiring and reselling this space.
If someone can sell something others may want to buy, it doesn't mean that what's being sold and bought is a property. There are people who are paying for some stars to have certain particular names in arbitrary private databases. "star-registration dot com" and such. IP addresses are essentially the very same thing.
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Töma