Someone from Craigslist posted to NANOG saying they were suffering from a lot of spam from unallocated IP address space. So I wrote a program which downloads the RIR allocation statistics tables, compiles a list of allocated /24 blocks, and scans Exim's logs for connections from unallocated space. It turns out we are not suffering from a lot of spam from unallocated IP address space - we get on average less than one bogon connection each day.
Tony Finch – blog