C Program to validate an IP address

In this program we will see how to validate an IP address using C. The IPv4 addresses are represented in dot-decimal notation. There are four decimal numbers (all are ranging from 0 to 255). These four numbers are separated by three dots.

An example of a valid IP is:

To validate the IP address we should follow these steps

  • Tokenize the string (IP address) using the dot “.” delimiter

  • If the sub strings are containing any non-numeric character, then return false

  • If the number in each token is not in range 0 to 255, then return false

  • If there are exactly three dots and four parts then it is a valid IP address

Example Code

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
int validate_number(char *str) {
   while (*str) {
      if(!isdigit(*str)){ //if the character is not a number, return
         return 0;
      str++; //point to next character
   return 1;
int validate_ip(char *ip) { //check whether the IP is valid or not
   int i, num, dots = 0;
   char *ptr;
   if (ip == NULL)
      return 0;
      ptr = strtok(ip, "."); //cut the string using dor delimiter
      if (ptr == NULL)
         return 0;
   while (ptr) {
      if (!validate_number(ptr)) //check whether the sub string is
         holding only number or not
         return 0;
         num = atoi(ptr); //convert substring to number
         if (num >= 0 && num <= 255) {
            ptr = strtok(NULL, "."); //cut the next part of the string
            if (ptr != NULL)
               dots++; //increase the dot count
         } else
            return 0;
    if (dots != 3) //if the number of dots are not 3, return false
       return 0;
      return 1;
int main() {
   char ip1[] = "";
   char ip2[] = "";
   char ip3[] = "192.800.100.1";
   char ip4[] = "125.512.100.abc";
   validate_ip(ip1)? printf("Valid
"): printf("Not valid
"); validate_ip(ip2)? printf("Valid
"): printf("Not valid
"); validate_ip(ip3)? printf("Valid
"): printf("Not valid
"); validate_ip(ip4)? printf("Valid
"): printf("Not valid
"); }


Not valid
Not valid

Updated on: 30-Jul-2019

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