Write a C program to check given string is valid IPv4 address or not

By | 03.12.2016

C program to check string is valid IPv4 address or not

Write a C program to check given string is valid IPv4 address or not.This program will read an IP address in String and check whether it is a valid IPv4 address or not.

According to Wikipedia, IPv4 addresses are canonically represented in dot-decimal notation, which consists of four decimal numbers, each ranging from 0 to 255, separated by dots, e.g.,


Following are steps to check whether a given string is valid IPv4 address or not:

step 1)

Parse string with “.” as delimiter using “strtok()” function.

e.g. ptr = strtok(str, DELIM);

step 2)
……..a) If ptr contains any character which is not digit then return 0
……..b) Convert “ptr” to decimal number say ‘NUM’
……..c) If NUM is not in range of 0-255 return 0
……..d) If NUM is in range of 0-255 and ptr is non-NULL increment “dot_counter” by 1
……..e) if ptr is NULL goto step 3 else goto step 1

step 3)

if dot_counter != 3 return 0 else return 1.

Below is the source code of the C program to check given string is valid IPv4 address or not which is successfully compiled and run on the windows system.The Output of the program is shown below.


#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')
            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)
        } 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[] = "";
    char ip2[] = "";
    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;

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About Tunde A

My name is Tunde Ajetomobi, a Tech Enthusiast and Growth Hacker. I enjoy creating helpful content that solves problem across different topics. Codezclub is my way of helping young aspiring programmers and students to hone their skills and find solutions on fundamental programming languages.

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