Just how many IPs are spamming, anyhow?

Yesterday, one of our architects popped into my office and we had a brief discussion about blocklists.  The topic shifted to how many IPs we have banned over the years.

The total number of available IPv4 IP addresses is 2³², or 4,294,967,296 different ones (4.3 billion).  He then said that the actual number is less than that since there are non-routable IP addresses.  I don't know the exact details, but that actually drops the number of IP addresses in half to 2.1 billion.  If anyone knows anything more about that, please let me know because I'm drawing a blank at the moment. 

Since October 2006, we have a list of IP addresses that we have banned from our network at one time or another.  We're only Exchange Hosted Services, we're not one of the big mail providers like Hotmail, Yahoo, or AOL.  Simply going by our own list of IPs from our own private list (ie, not a 3rd party list like Spamhaus), we have blocked 1.5% of the total available IP addresses, or 3% of the IP addresses if you use the scaled down figure.

That's a heck of a lot of IPs to ban in a relatively short period of time.