A curious person in 2012 took over 420,000 routers, uninterrupted power supplies, printers and other internet-enabled devices whose passwords nobody bothers to change from the default to see who used the internet. His probes created a visual mosaic of the World Wide Web. He then quietly deleted his code. His deed is called the Carna Botnet.
He found that of the then 4.3 billion possible IPv4 addresses about 2.3 billion were not used.
4.3 billion is 4.3 x 109.