Comment by bigiain

It is definitely "literally impossible" if your acceptable false positive and false negative rates are zero.

Having said that, vanishingly few companies/projects require that. For probably 99+% of websites, just using publicly available GeoIP databases to block countries will work just fine, so long as you don't pretend to yourself that North Korean or Chinese or Russian (or wherever) web users (or attackers) cannot easily get around that. And you'll also need to accept that occasionally a "local/wanted" user will end up with an IP address that gets blocked due to errors in the database.

I worked on a project a decade or so back where we needed to identify which (Australian) state a website user was in, to correctly display total driveaway prices including all state taxes/charges (stamp duty, ctp insurance, and registration) for new cars. The MaxMind GeoIP database was not all that accurate at a state or city level, especially for mobile devices with CGNATed IP addresses. We ended up with "known errors and estimates of error rates", and a way for our Javascript to detect some of the known problems (like Vodafone's national CGNAT IP addresses) and popped up a "We detected you're in NSW, and are displaying NSW pricing. Click here to change state." message where we could, and got legal signoff that we could claim "best effort" at complying with the driveway price laws. 100% compliance with the laws as-written was "literally impossible" with zero error rates.