use strict;
use warnings;

my @ips = qw(1.2.3.4 1.2.3.5 1.2.3.7);

my $first = shift @ips;
my @identicals = split m/\./, $first;
for (@ips){
	my @b = split m/\./;
	for my $i (0 .. $#identicals){
		if ($b[$i] ne $identicals[$i]){
			splice @identicals, $i;
			last;
		}
	}
}

my @rest = (0) x (4 - @identicals);
my $net = join('.', @identicals, @rest) . '/' . (8 * @identicals);
print "$net\n";