OmniPing was developed for network administrators and engineers for the
purpose of network analysis and, in particular, IP address tracking,
recovery, and assignment. OmniPing can be used to verify what machines
are running on a network, to find unused IP addresses from machines that
are no longer attached to the network, or to help in organization and
planning of network restructuring. In OmniPing Professional, the result
of each OmniPing "run" can be saved to database. There is even a nice
report to show free and used IP addresses after a number of runs have
been saved!
http://manasoft.com/manasoft/omniping_pro.html