修改下列代码增加可以按需分配构造超网功能：import javaio; public class ffdg param args public static void mainString argsthrows IOException TODO Auto-generated method stub String ip; 用于接收要划分的IP int s;

import java.io.*;

public class ffdg { /** * @param args */ public static void main(String[] args)throws IOException { // TODO Auto-generated method stub String ip; //用于接收要划分的IP
int s; //用于接收以要划分的子网数
int n; //输入子网数对应的二进制位数

    System.out.println("---------------划分子网---------------");  
    //输入要划分的IP地址:ip  
    System.out.println("请输入要划分的IP地址(格式为：x1.x2.x3.x4)：");  
    System.out.print("→:");  
    InputStreamReader isr = new InputStreamReader(System.in);  
    BufferedReader br = new BufferedReader(isr);  
    ip = br.readLine();  

    //截取x1  
    int index1 = ip.indexOf('.');  
    String st1 = ip.substring(0,index1);  
    int x1=Integer.parseInt(st1);//把String类型的st转化成int型  

    //截取x2  
    int index2= ip.indexOf('.',index1+1);  
    String st2 = ip.substring(index1+1,index2);  
    int x2=Integer.parseInt(st2);//把String类型的st转化成int型  

    //截取x3  
    int index3= ip.indexOf('.',index2+1);  
    String st3 = ip.substring(index2+1,index3);  
    int x3=Integer.parseInt(st3);//把String类型的st转化成int型  

    //截取x4  
    //A类  
    if(x1>0&&x1<=126){  
        System.out.println("本IP地址能划分划分的最大子网数为: "+((int)Math.pow(2,24-2)-2));  
    }  
    //B类  
    if(x1>=128&&x1<=191){  
        System.out.println("本IP地址能划分划分的最大子网数为: "+((int)Math.pow(2,16-2)-2));  
    }  
    //C类  
    if(x1>=192&&x1<=223){  
        System.out.println("本IP地址能划分划分的最大子网数为: "+((int)Math.pow(2,8-2)-2));  
    }  

    //输入要划分的子网数数:s  
    System.out.println("请输入要划分的子网数：");  
    System.out.print("→:");  

    String subnet_num= br.readLine();  
    s = Integer.parseInt(subnet_num);  
    br.close();  


    //求所输入子网数对应的二进制位数:n  
    int maxSubnets = 0;
    int subnetMask = 0;
    int maxHosts = 0;
    for(n=1; n<=30; n++) {
        int subnets = (int)Math.pow(2, n);
        int hosts = (int)Math.pow(2, 32-n)-2;
        if(subnets >= s && maxSubnets < subnets) {
            maxSubnets = subnets;
            subnetMask = 0xffffffff << (32-n);
            maxHosts = hosts;
        }
    }

    System.out.println("--------------------------------------");  
    //求子网掩码  
    System.out.println("子网掩码为：");  
    System.out.println(((subnetMask >> 24) & 0xff) + "." + ((subnetMask >> 16) & 0xff) + "." + ((subnetMask >> 8) & 0xff) + "." + (subnetMask & 0xff));
    System.out.println("每网段主机数：");  
    System.out.println(maxHosts);

    //输出各子网IP范围  
    System.out.println("各子网IP范围如下：");  
    for(int i=0; i<maxSubnets; i++) {
        int subnetId = i << (32-n);
        System.out.print("<"+(i+1)+">"+" ");
        System.out.println(((subnetId >> 24) & 0xff) + "." + ((subnetId >> 16) & 0xff) + "." + ((subnetId >> 8) & 0xff) + "." + ((subnetId & 0xff)+1) + "---" + ((subnetId >> 24) & 0xff) + "." + ((subnetId >> 16) & 0xff) + "." + ((subnetId >> 8) & 0xff) + "." + ((subnetId & 0xff)+(int)Math.pow(2,32-n)-2));
    }

    System.out.println("子网划分完毕");  
}