Checking String is Valid IPv4 address or not

/*C program to check given string is a valid IPv4 address or not.*/

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define DELIM "."

/* return 1 if string contain only digits, else return 0 */
int valid_digit(char* ip_str)
{
    while (*ip_str) {
        if (*ip_str >= '0' && *ip_str <= '9')
            ++ip_str;
        else
            return 0;
    }
    return 1;
}

/* return 1 if IP string is valid, else return 0 */
int is_valid_ip(char* ip_str)
{
    int i, num, dots = 0;
    char* ptr;

    if (ip_str == NULL)
        return 0;

    ptr = strtok(ip_str, DELIM);

    if (ptr == NULL)
        return 0;

    while (ptr) {

        /* after parsing string, it must contain only digits */
        if (!valid_digit(ptr))
            return 0;

        num = atoi(ptr);

        /* check for valid IP */
        if (num >= 0 && num <= 255) {
            /* parse remaining string */
            ptr = strtok(NULL, DELIM);
            if (ptr != NULL)
                ++dots;
        }
        else
            return 0;
    }

    /* valid IP string must contain 3 dots */
    if (dots != 3)
        return 0;
    return 1;
}

// Driver program to test above functions
int main()
{
    char ip1[] = "128.0.0.251";
    char ip2[] = "128.16.100.1";
    char ip3[] = "128.512.100.1";
    char ip4[] = "125.512.200.cde";

    is_valid_ip(ip1) ? printf("Valid\n") : printf("Not valid\n");
    is_valid_ip(ip2) ? printf("Valid\n") : printf("Not valid\n");
    is_valid_ip(ip3) ? printf("Valid\n") : printf("Not valid\n");
    is_valid_ip(ip4) ? printf("Valid\n") : printf("Not valid\n");

    return 0;
}

Output:

    Valid
    Valid
    Not valid
    Not valid