java获取子网掩码

可以使用Java中的InetAddress和NetworkInterface类来获取子网掩码。

以下是一个示例代码：

import java.net.InetAddress;
import java.net.NetworkInterface;
import java.net.SocketException;
import java.util.Enumeration;

public class SubnetMaskExample {
  public static void main(String[] args) throws SocketException {
    Enumeration<NetworkInterface> interfaceList = NetworkInterface.getNetworkInterfaces();
    while (interfaceList.hasMoreElements()) {
      NetworkInterface iface = interfaceList.nextElement();
      Enumeration<InetAddress> addrList = iface.getInetAddresses();
      while (addrList.hasMoreElements()) {
        InetAddress address = addrList.nextElement();
        if (!address.isLinkLocalAddress() && !address.isLoopbackAddress() && address instanceof java.net.Inet4Address) {
          System.out.println("IP地址：" + address.getHostAddress());
          System.out.println("子网掩码：" + getSubnetMask(iface));
        }
      }
    }
  }

  public static String getSubnetMask(NetworkInterface iface) throws SocketException {
    Enumeration<InetAddress> addrList = iface.getInetAddresses();
    while (addrList.hasMoreElements()) {
      InetAddress address = addrList.nextElement();
      if (!address.isLinkLocalAddress() && !address.isLoopbackAddress() && address instanceof java.net.Inet4Address) {
        short prefixLength = iface.getInterfaceAddresses().get(0).getNetworkPrefixLength();
        int mask = 0xffffffff << (32 - prefixLength);
        byte[] bytes = new byte[] {(byte) (mask >>> 24), (byte) (mask >> 16 & 0xff), (byte) (mask >> 8 & 0xff), (byte) (mask & 0xff)};
        try {
          return InetAddress.getByAddress(bytes).getHostAddress();
        } catch (Exception e) {
          e.printStackTrace();
        }
      }
    }
    return null;
  }
}

该代码会遍历所有的网络接口，获取每个接口的IP地址和子网掩码。需要注意的是，获取子网掩码需要根据网络前缀长度进行计算，这里使用了Java中的位运算来实现。