请用Java计算子网掩码

Java可以通过NetworkInterface、InterfaceAddress这两个类来实现获取子网掩码前缀的功能，然后通过运算就能得到子网掩码[^1]。下面是一个获取子网掩码的例子：

import java.net.InetAddress;
import java.net.InterfaceAddress;
import java.net.NetworkInterface;
import java.util.Enumeration;
import java.util.List;

public class SubnetMaskCalculator {
    public static void main(String[] args) throws Exception {
        Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
        while (interfaces.hasMoreElements()) {
            NetworkInterface networkInterface = interfaces.nextElement();
            if (networkInterface.isLoopback() || !networkInterface.isUp()) {
                continue;
            }
            List<InterfaceAddress> interfaceAddresses = networkInterface.getInterfaceAddresses();
            for (InterfaceAddress interfaceAddress : interfaceAddresses) {
                InetAddress inetAddress = interfaceAddress.getAddress();
                if (!(inetAddress instanceof java.net.Inet4Address)) {
                    continue;
                }
                short prefixLength = interfaceAddress.getNetworkPrefixLength();
                int mask = 0xffffffff << (32 - prefixLength);
                byte[] bytes = new byte[]{(byte) (mask >>> 24), (byte) (mask >> 16 & 0xff), (byte) (mask >> 8 & 0xff), (byte) (mask & 0xff)};
                InetAddress netmask = InetAddress.getByAddress(bytes);
                System.out.println("Interface name : " + networkInterface.getDisplayName());
                System.out.println("IP address : " + inetAddress.getHostAddress());
                System.out.println("Subnet mask : " + netmask.getHostAddress());
            }
        }
    }
}