You mean to find out if the arbitrary IP is in the range given by the CIDR notation (such as is 192.168.1.2 in the range 10.0.2.0/24)?

*sigh* I wish I had found this post earlier today, when I wrote my own function to do this (which someone else has probably already come up with, and easier, probably):

sub inblock {
    my($block, $cidr, $target) = @_;
    my $result = 1, $machinebits = (32 - $cidr);
    my $lip1 = unpack("N", pack("c4", split(/\D/, $block,  4)));
    my $lip2 = unpack("N", pack("c4", split(/\D/, $target, 4)));

    $result = 0
        if (($lip1 >> $machinebits) != ($lip2 >> $machinebits));
    return($result);
}
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