I've been attempting to use Firewall PAPI as a reference for a project of mine (using the Packet Filtering API.) However, both PAPI and my project cannot block specific IPs: it can only block all IPs by leaving both source and destination IPs set to "all." Surely this was not a problem when it was released, as it would be pretty useless to have a packet filter that can't filter packets.

Has something changed since then? I'm using Windows XP SP3, which to my knowledge works with the API. I'm sure I'm putting the right IPs in (pinging a website to test) so I don't know what else it could be. Am I missing something?