my %conversations;

while (<DATA>) {
   my ($src, $dst) = (split)[0, 2];
   $src = pack('C4n', split(/[.:]/, $src));
   $dst = pack('C4n', split(/[.:]/, $dst));
   my $key = $src lt $dst ? "$src$dst" : "$dst$src";
   push(@{$conversations{$key}}, $_);
}

foreach (values %conversations) {
   print(@$_);
}