java 如何将ip 1.2.3.4 变成1.2.3.0/24

如果你想手动实现而不使用第三方库，你可以按照如下的方式进行。以下是一个手动实现的简单示例：

import java.net.InetAddress;
import java.net.UnknownHostException;

public class IpToCidrConverter {
   
    public static void main(String[] args) {
   
        String ipAddress = "1.2.3.4";
        int subnetPrefixLength = 24;

        try {
   
            String cidrNotation = convertToCidr(ipAddress, subnetPrefixLength);
            System.out.println("CIDR Notation: " + cidrNotation);
        } catch (UnknownHostException e) {
   
            e.printStackTrace();
        }
    }

    private static String convertToCidr(String ipAddress, int subnetPrefixLength) throws UnknownHostException {
   
        InetAddress inetAddress = InetAddress.getByName(ipAddress);
        byte[] address = inetAddress.getAddress();

        // Convert the IP address bytes to an integer
        int ipAddressInt = 0;
        for (byte b : address) {
   
            ipAddressInt = (ipAddressInt << 8) | (b & 0xFF);
        }

        // Create a bitmask based on the subnet prefix length
        int subnetMask = 0xFFFFFFFF << (32 - subnetPrefixLength);

        // Apply the bitmask to the IP address to get the network address
        int networkAddress = ipAddressInt & subnetMask;

        // Convert the network address back to byte array
        byte[] networkAddressBytes = new byte[4];
        for (int i = 0; i < 4; i++) {
   
            networkAddressBytes[i] = (byte) ((networkAddress >> (24 - i * 8)) & 0xFF);
        }

        // Format the CIDR notation
        StringBuilder cidrNotation = new StringBuilder();
        for (int i = 0; i < 4; i++) {
   
            cidrNotation.append((networkAddressBytes[i] & 0xFF));
            if (i < 3) {
   
                cidrNotation.append(".");
            }
        }
        cidrNotation.append("/").append(subnetPrefixLength);

        return cidrNotation.toString();
    }
}

这个手动实现的代码与使用第三方库的版本相似，但是它直接处理了CIDR表示法的格式。你可以根据需要调整输入参数。请注意，这个示例假设输入的IP地址和子网前缀长度是有效的。在实际应用中，你可能需要添加一些输入验证。