use constant MAXIP => 2**(8*4)-1;

sub quad2num
{ my $num = 0;
  my @bytes = split('\.', shift);
  map { $num = 256 * $num + $_ } @bytes;
  die "IP number $num out of range" unless ($num >= 0 and $num <= MAXIP);
  return $num;
}

sub num2quad
{ my $num = shift;
  my @bytes = ();
  return $num if $num =~ /^\d+\.\d+\.\d+\.\d+$/;

  unless ($num >= 0 and $num <= MAXIP)
  { warn "IP number $num out of range 0..$maxip"; return undef }
  foreach (1..4)
  { unshift @bytes, $num % 256;
    $num >>= 8;  # $num /= 2**8;
  }
  return join('.',@bytes);
}