sub store_subnet {
    my ( $ip, $len, $comment ) = @_;
    my $ipn = unpack "N", Socket::inet_aton $ip;
    $len =~ s/^\///;
    my $maskn = 0xFFFFFFFF << ( 32 - $len );
    $ipn = $ipn & $maskn;
    $ip = Socket::inet_ntoa pack "N", $ipn;
    $subnets{"$ip,$len,$comment"} = { ipn => $ipn, maskn => $maskn, name => $comment };
}