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bool verifyIP(char* ip)
{

int ippartN=0;
int count=0;
//be in the form of 255.255.255.255
//1.read one char, a.test whether it is digit: if yes, concat it to the string variable;if no, return 0;
//2.when reaching an ".", end composing the string variable, convert it to an integer and test wehther it is between 0-255, if yes, continue with next step, if no, return 0
//3.if the string is a numberic string between 0-255, increase the count with 1
//4.continue step 1-3 till end of the string
//5.test whether count=4, if yes, return 1, otherwise return 0;
char current=0;
char prechar=0;
if(!ip) return 0;
while(*ip)
{
current=*ip;

if(current=='.')
{

if(prechar==current)
return 0;
ippartN=0;
count++;
prechar=current;
if(count>4) return 0;
ip++;
continue;
}
else{
if((current<'0')||(current>'9'))
return 0;

ippartN=ippartN*10+current-'0';
prechar=current;
}
ip++;
}
if((ippartN<0)||(ippartN>255))
return 0;
count++;
if(count==4) return 1;
return 0;

}

bool check_ip(char * IP)
{
	//char IP[16] = "255.255.255.255";
	if(strlen(IP) > 16) 
		return false;
	
	char * range = strtok(IP, ".");
	short c = 0;
	while ((range != 0) && (c < 4)) {
	  if((strlen(range) <= 3) && (atoi(range) >= 0) && (atoi(range) <= 255)) ++c;
	  else break;
	  
	  range = strtok(0, ".");
	}
	
	return (c == 4);
}

Or you could just use a regular expression....

(?:(?: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]?)

static bool IsValidIpAddress( string address )
        {
            if (address == null)
                return false;

            string[] numbers = address.Split('.');

            if (numbers.Length != 3)
                return false;

            foreach (string current in numbers)
            { 
                int num = 0;

                if (!int.TryParse(current, out num))
                    return false;
                
                if (num < 0 || num > 255)
                    return false;
            }

            return true;
        }

numbers.Length != 4

not sure if atoi does error handling like Integer.valueof(String s)...
I mean ...atoi("3j4")...you will not get any exception...

so we need to check that all the 3 chars are valid digits...maybe that time also check permissible digit values for each char...like 0-2...0-5...0-5...small optimization