use strict;
use warnings;

my $subnet_size = 29;
my $subnet_mask = pack('B*', ('1' x $subnet_size)
                           . ('0' x (32-$subnet_size)));

my @ip_xfers = (
   [ '10.0.0.1', 555555 ],
   [ '10.2.1.6', 444444 ],
   [ '10.6.2.8', 333333 ],
   [ '10.2.1.2', 222222 ],
);

my %subnet_xfers;
foreach (@ip_xfers) {
   my ($ip, $xfer) = @$_;
   my $packed_ip = pack('C4', split(/\./, $ip));
   my $subnet = $packed_ip & $subnet_mask;
   $subnet_xfers{$subnet} ||= 0;
   $subnet_xfers{$subnet} += $xfer;
}

foreach my $subnet (sort keys %subnet_xfers) {
   my $subnet_ip = join('.', unpack('C4', $subnet));
   my $xfer = $subnet_xfers{$subnet};
   print("$subnet_ip/$subnet_size: $xfer\n");
}

##</code><code>##

10.0.0.0/29: 555555
10.2.1.0/29: 666666
10.6.2.8/29: 333333