题目链接：~~~

用正则表达式写：

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class hdu_IP计算2206 {

	public static void main(String[] args) {
		
		Scanner sc= new Scanner(System.in);
		String s;
		while(sc.hasNext()){
			s = sc.nextLine();
Pattern p = Pattern.compile("^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$");
			Matcher m = p.matcher(s);
			if(m.find())
				System.out.println("YES");
			else
				System.out.println("NO");
		}

	}

}

C/C++：

#include<stdio.h>
int main(){
	char str[102],tmp[102],a[4],b[4],c[4],d[4];
	while(gets(str))
	{
		tmp[0]='\0';
		if(sscanf(str,"%3[0-9].%3[0-9].%3[0-9].%3[0-9]%s",a,b,c,d,tmp)==4&&!tmp[0])
		{
			int a,d;
			sscanf(str,"%d.%d.%d.%d",&a,&b,&c,&d);
			if(a<256&&b<256&&c<256&&d<256)
				puts("YES");
			else
				puts("NO");
		}
		else
			puts("NO");
	}
    return 0;
}