Using Classic ASP, I want to check if a given IP is within an IP range in CIDR (e.g. 192.168.0.0/24).

Some examples would be:

• 192.168.0.1 belongs to 192.168.0.0/24? (yes)
• 192.168.0.100 belongs to 192.168.0.0/24? (yes)
• 192.168.0.130 belongs to 192.168.0.0/25? (no! this one is 192.168.0.0-192.168.0.127)

And, of course, going out with the bitmasks from /32 to /0 (well the latter one should say “yes” to everything)

The actual use case is to check if a CloudFlare-proxied connection claiming CF-Connecting-IP header is actually coming from CloudFlare by the list they provide of ipv4 endpoints.

I’ve searched far and wide, and basically what I found in Stackoverflow was about returning IP ranges, or regex-validating IPs, so they either not help in my case, or were way cumbersome to base my code off.