如何识别字符串是一个IP地址

importjava.io.BufferedReader;
importjava.io.IOException;
importjava.io.InputStreamReader;

public class IpTest {
public static void main(String[] args) {
new IpTest().go();
}

/**
* 程序主要逻辑
*/

public void go() {
String IP = null;
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
try {
IP = br.readLine();
} catch (IOException ex) {
ex.printStackTrace();
}

//String IP="192.168.0.100/255.255.255.0";
IP = IP.replace(" ", "");
String[] temp = IP.split("/");
String[] sIP = temp[0].split("\\.");

/**
* 判断输入格式是否正确
*/
if (temp.length < 2) {
System.out.println("对不起，输入格式错误");
System.exit(0);
}

int Mask = Integer.parseInt(temp[1]);
int[] ip = new int[4];

for (int i = 0; i < sIP.length; i++) {
ip[i] = Integer.parseInt(sIP[i]);
}

/**
* IP地址各段不能大于255
*/

for (int i = 0; i < 4; i++) {
if (ip[i] > 255) {
System.out.println("对不起，IP输入错误");
System.exit(0);
}
}

/**
* 判断IP地址所属分类
*/

int ipclass = 0;

if (ip[0] < 127) {
System.out.println("A类地址");
ipclass = 1;
} else if (ip[0] < 192) {
System.out.println("B类地址");
ipclass = 2;
} else if (ip[0] < 224) {
System.out.println("C类地址");
ipclass = 3;
}

/**
* 判断子网掩码是否输入正确
*/

if (Mask < 8 * ipclass) {
System.out.println("对不起，子网掩码输入错误");
System.exit(0);
}

int b = 0;
int a = Mask % 8;

b = Mask / 8;

StringBuffer sb = new StringBuffer();

for (int i = 0; i < a; i++)
sb.append('1');
for (int i = 0; i < 8 - a; i++)
sb.append('0');

int end = Integer.parseInt(sb.toString(), 2);

ip[b] = ip[b] & end;
for (int i = b + 1; i < 4; i++)
ip[i] = 0;

for (int i = 0; i < 4; i++) {
System.out.print(ip[i]);
if (i != 3)
System.out.print(".");
}

}

}