判断一个ip地址合法性（基础c，不用库函数）

#include <stdio.h>
int judge(char *strIp);
int main()
{
    char a[20];
    while(1)
    {
        scanf("%s", a);
        if(0 == judge(a))
        {
            printf("this is a correct ip
");
        }
    }
}

int judge(char *strIp)
{
　　 if(NULL == strIp)
　　　{
　　　　　　return -1;
　　　}

    //IP must start with 0-9
    if(*strIp > '9' || *strIp < '0')
    {
        return -1;
    }
    int num = 0;
    int dotNum = 0;
    do
    {
        if(*strIp >= '0' && *strIp <= '9')
        {
            num = 10 * num + *strIp - '0';
        }
        else if(('.' == *strIp) || ('' == *strIp))
        {
            //filter the string including '..' or ending with '.'
            if('.' == *(strIp - 1))
            {
                return -1;
            }
            //filter the section number large than 255 or little than 0
            if(num < 0 || num > 255)
            {
                return -1;
            }
            num = 0;
            //record the number of right dots
            if('.' == *strIp)
            {
                dotNum++;
            }
        }
        else
        {
            //Can't have chars excluding '0' - '9' and '.'
            return -1;
        }
    }while(*(strIp++))
    
    if(3 == dotNum)
    {
        // ip is correct
        return 0;
    }
    else
    {
        //the string is too short for ip
        return -1;
    }
    
}