BuduIP is a free IP scanner that scans all IP addresses in a network, and finds out which IP addresses are used, and which ones are vacant. You just need to provide IP Range that you want to scan, and BuduIP will scan all IP addresses within that range. You can export the generated report to know which IP addresses are vacant.
To use this free IP scanner, just provide the IP address range. For example, in the screenshot below, the starting IP address has been provided as 192.168.1. This free IP Scanner then generates all IP addresses that start with 192.168.1, and pings them to see if they exist. If it does not gets response back from some IP address, it classifies it as vacant, and classifies other as Taken.
This free IP Scanner is quite useful in large organizations where network administrators want to track which IP addresses have been taken, and which are available. It can even be used for home users to see that no unatuhorized user is connected to their network. Also check out free VPN software.
Download BuduIP here.