1、子網掩碼地址轉換為網絡位長度

// 如 255.255.255.0 對應的網絡位長度為 24
func SubNetMaskToLen(netmask string) (int, error) {
	ipSplitArr := strings.Split(netmask, ".")
	if len(ipSplitArr) != 4 {
		return 0, fmt.Errorf("netmask:%v is not valid, pattern should like: 255.255.255.0", netmask)
	}
	ipv4MaskArr := make([]byte, 4)
	for i, value := range ipSplitArr {
		intValue, err := strconv.Atoi(value)
		if err != nil {
			return 0, fmt.Errorf("ipMaskToInt call strconv.Atoi error:[%v] string value is: [%s]", err, value)
		}
		if intValue > 255 {
			return 0, fmt.Errorf("netmask cannot greater than 255, current value is: [%s]", value)
		}
		ipv4MaskArr[i] = byte(intValue)
	}

	ones, _ := net.IPv4Mask(ipv4MaskArr[0], ipv4MaskArr[1], ipv4MaskArr[2], ipv4MaskArr[3]).Size()
	return ones, nil
}

2、網絡位長度轉換為子網掩碼地址

// 如 24 對應的子網掩碼地址為 255.255.255.0
func LenToSubNetMask(subnet int) string {
	var buff bytes.Buffer
	for i := 0; i < subnet; i++ {
		buff.WriteString("1")
	}
	for i := subnet; i < 32; i++ {
		buff.WriteString("0")
	}
	masker := buff.String()
	a, _ := strconv.ParseUint(masker[:8], 2, 64)
	b, _ := strconv.ParseUint(masker[8:16], 2, 64)
	c, _ := strconv.ParseUint(masker[16:24], 2, 64)
	d, _ := strconv.ParseUint(masker[24:32], 2, 64)
	resultMask := fmt.Sprintf("%v.%v.%v.%v", a, b, c, d)
	return resultMask
}