// listIPsInRange will list all the ips in range, the range will be the whole network
// when begin and end is empty.
func listIPsInRange(cidr string, begin string, end string) (ips []string) {
	// convert string to IPNet struct
	_, ipv4Net, _ := net.ParseCIDR(cidr)
	mask := binary.BigEndian.Uint32(ipv4Net.Mask)

	var start uint32
	var finish uint32
	if begin != "" && end != "" {
		beginIP := net.ParseIP(begin)
		beginB := beginIP[12:]           // But the IP returned by ParseIP is 16 bytes, I have to split it.
		endIP := net.ParseIP(end)
		endB := endIP[12:]                // But the IP returned by ParseIP is 16 bytes, I have to split it.
		start = binary.BigEndian.Uint32(beginB)   
		finish = binary.BigEndian.Uint32(endB)
	} else {
		start = binary.BigEndian.Uint32(ipv4Net.IP)   // The IPNet.IP can transfer easily like this.
		finish = (start & mask) | (mask ^ 0xffffffff)
	}

	// loop through addresses as uint32
	for i := start; i <= finish; i++ {
		// convert back to net.IP
		ip := make(net.IP, 4)
		binary.BigEndian.PutUint32(ip, i)
		fmt.Println(ip)
		ips = append(ips, ip.String())
	}

	if begin != "" && end != "" {
		return ips
	}

	// Ignore network address and broadcast address
	return ips[1 : len(ips)-1]
}