First off, this subject has been discussed before but I felt that there's enough current interest in this board and on other boards here at WebmasterWorld alone, to warrant a fresh top-down discussion of the subject. We'll see if our moderators agree.
The list of CIDRs below was compiled from the Iana IPv4 Address Space Registry report [iana.org]. The list is a compact version of all Allocated non-ARIN /8 blocks (from APNIC, RIPE NCC, AFRINIC, and LACNIC). For example, 220.127.116.11/7 actually merges 18.104.22.168/8 and 22.214.171.124/8 into a single CIDR. The largest block in this list is 126.96.36.199/4 which merges the 188.8.131.52 through 184.108.40.206 address range.
Some of the CIDR's below merge blocks from different registries e.g. combining blocks from both RIPE NCC and APNIC. As such, this does not in any way represent an approach surgical enough to differentiate blocks in one RIR from blocks in another (let alone blocks representing specific countries). The goal here is to arrive at a blocking strategy that keeps people and bots from outside North America off your site.
It should also be noted that the list below is only intended as a good first step where blocking is concerned. There are many holes in the Legacy blocks that this step does not address and proxies are another whole topic of ingress. The intention here is to succinctly narrow the scope of the task with as little effort as possible.
One tangible benefit of this approach can be seen in the 220.127.116.11/5 range which blocks 18.104.22.168 to 22.214.171.124. This CIDR contains some AWS and Rackspace ranges (and probably other server farms as well). Blocking this range means you don't have to identify and separately block those server farm ranges.
1.This list is helpful to those looking for a starting point 2.That, if there's a mistake in the list above, that the moderators will see fit to correct the list when the mistake is identified so that the first post can reflect accurate and up-to-date information. 3.That this discussion can move forward with new ranges outside the Allocated blocks to help expand this list even further. Anyone want to block the UK Ministry of Defence (sic)? That /8 block and others are omitted here in this initial list because they are Legacy blocks.
And last for now. It is possible to further reduce the above list to a series of Regular Expressions which would be even more condensed than the list above. For those with access to a rewrite module (Apache or IIS) this list would be valuable but I'll leave up to an expert in that arena to post the list if they care to. I hope this helps someone and can save them the time I (and many others) have spent whittling down the world a bit.