Block CIDR Country Range

[gatekeeper]

I want to block all other IPs and allow only:

(1) USA
(2) Canada
(3) Europe
(4) Australia

but I want to do that in *the most efficient way*, instead of adding thousands IP ranges into my IPTables firewall.

This link would show was relevant to what I want to do, but it is not enough.

Please any help would be appreciated.

Thanks.

[gatekeeper]

Correction to the above:

This link was relevant to what I want to do, but it is not enough.