while (<IN>) {
	if (/^(\d+)\.(\d+)\.(\d+)\.(\d+)/) {
		my $a = $1;
		my $b = $2;
		my $c = $3;	
		my $d = $4;
		my $ip = "$a.$b.$c.$d";
		my $classc = "$a.$b.$c";
		my $classb = "$a.$b";
		my $classa = "$a";
		push(@{$hash{$classa}{$classb}{$classc}}, $ip);
	}
}



for my $a ( keys %hash ) { 
	for my $b ( keys %{$hash{$a}} ) { 
		for my $c ( keys %{$hash{$a}{$b}} ) { 
			for my $ip ( @{$hash{$a}{$b}{$c}} ) { 
				print "$ip\n";
			} 
		} 
	} 
} 


use Data::Dumper; print Dumper \%hash;