Program to validate an IP address

Difficulty Level : Medium
Last Updated : 09 Nov, 2020

Write a program to Validate an IPv4 Address.
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., 172.16.254.1

Following are steps to check whether a given string is a 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

C++

// Program to check if a given 
// string is valid IPv4 address or not
#include <bits/stdc++.h>
using namespace std;
#define DELIM "."
 
/* function to check whether the 
   string passed is valid or not */
bool valid_part(char* s)
{
    int n = strlen(s);
     
    // if length of passed string is 
    // more than 3 then it is not valid
    if (n > 3)
        return false;
     
    // check if the string only contains digits
    // if not then return false
    for (int i = 0; i < n; i++)
        if ((s[i] >= '0' && s[i] <= '9') == false)
            return false;
    string str(s);
     
    // if the string is "00" or "001" or 
    // "05" etc then it is not valid
    if (str.find('0') == 0 && n > 1)
        return false;
    stringstream geek(str);
    int x;
    geek >> x;
     
    // the string is valid if the number
    // generated is between 0 to 255
    return (x >= 0 && x <= 255);
}
 
/* return 1 if IP string is 
valid, else return 0 */
int is_valid_ip(char* ip_str)
{
    // if empty string then return false
    if (ip_str == NULL)
        return 0;
    int i, num, dots = 0;
    int len = strlen(ip_str);
    int count = 0;
     
    // the number dots in the original 
    // string should be 3
    // for it to be valid
    for (int i = 0; i < len; i++)
        if (ip_str[i] == '.')
            count++;
    if (count != 3)
        return false;
     
    // See following link for strtok()
   
    char *ptr = strtok(ip_str, DELIM);
    if (ptr == NULL)
        return 0;
 
    while (ptr) {
 
        /* after parsing string, it must be valid */
        if (valid_part(ptr)) 
        {
            /* parse remaining string */
            ptr = strtok(NULL, ".");
            if (ptr != NULL)
                ++dots;
        }
        else
            return 0;
    }
 
    /* valid IP string must contain 3 dots */
    // this is for the cases such as 1...1 where 
    // originally the no. of dots is three but 
    // after iteration of the string we find 
    // it is not valid
    if (dots != 3)
        return 0;
    return 1;
}
 
// Driver code
int main()
{
    char ip1[] = "128.0.0.1";
    char ip2[] = "125.16.100.1";
    char ip3[] = "125.512.100.1";
    char ip4[] = "125.512.100.abc";
    is_valid_ip(ip1) ? cout<<"Valid\n" : cout<<"Not valid\n";
    is_valid_ip(ip2) ? cout<<"Valid\n" : cout<<"Not valid\n";
    is_valid_ip(ip3) ? cout<<"Valid\n" : cout<<"Not valid\n";
    is_valid_ip(ip4) ? cout<<"Valid\n" : cout<<"Not valid\n";
    return 0;
}

Output

Valid
Valid
Not valid
Not valid

This article is compiled by Narendra Kangralkar. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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