近期做了一個遠端防火牆的專案，需要根據送過來的IP和子網掩碼位數計運算元網，做了最簡單的實現：

  /**
   * 根據掩碼位數獲取子網掩碼
   * 
   * @param maskBit
   * @return
   */
  public static String getMaskByBit(int maskBit) {
    String maskIp = "";

    if (maskBit > 32 || maskBit < 1)
      return maskIp;

    String maskBinary = "";
    for (int i = 0; i < 32; i++) {
      if (i < maskBit)
        maskBinary += "1";
      else {
        maskBinary += "0";
      }
    }

    for (int i = 0; i < 4; i++) {
      maskIp += "." + (Integer.parseInt(maskBinary.substring(8 * i, 8 * (i + 1)), 2));
    }
    return maskIp.replaceFirst(".", "");

  }


  /**
   * 
   * @Title: getNetSegmentByIp
   * @Description: 根據IP計算出子網
   * @param: @param
   *         ip
   * @param: @return
   *         子網
   * @return: String
   * @throws null
   */
  public static String getNetSegmentByIp(String ip) {
    String[] ip_mask = ip.split("/");
    String[] ipv4 = ip_mask[0].split("\\.");
    int netBit = Integer.parseInt(ip_mask[1]);
    String mask = getMaskByBit(netBit);
    String[] maskv4 = mask.split("\\.");
    String result = "";
    for (int i = 0; i < maskv4.length; i++) {
      int ipBit = Integer.parseInt(ipv4[i]);
      int maskBit = Integer.parseInt(maskv4[i]);
      result += "." + (ipBit & maskBit);
    }
    return result.replaceFirst(".", "") + "/" + netBit;
  }

 

使用時，直接 getNetSegmentByIp("10.1.121.12/16")即可