A security research group has completed an Internet census revealing interesting facts on usage. According to the abstract by the crew from Carna Botnet,
While playing around with the Nmap Scripting Engine (NSE) we discovered an amazing number of open embedded devices on the Internet. Many of them are based on Linux and allow login to standard BusyBox with empty or default credentials. We used these devices to build a distributed port scanner to scan all IPv4 addresses. These scans include service probes for the most common ports, ICMP ping, reverse DNS and SYN scans. We analyzed some of the data to get an estimation of the IP address usage.
Their conclusions were the result of data from the 420,000-node botnet taken over a 24 hour period. The GIF above is produced by Carma Botnet and shows an animated world map of 24 hour relative average utilization of IPv4 addresses observed using ICMP ping requests.
The Internet Census 2012 report can be found on Bitbucket.