How to increment an IP address in a loop? [C]

Question

I know this is pretty simple for most of you but I am trying to increment an ip address by +1 in a loop.

Example:

for(double ip = 1.1.1.1; ip < 1.1.1.5; ip++)
{
printf("%f", ip);
}

Basically all I am trying to do is increment the ip by +1 in a for loop. I don't know what type of variable to store the ip in and don't know how to increment it. Whenever I run the program I get an error saying that the number has too many decimal points. I've also seen on the internet that you have to store ip's in a character array, but you cannot increment a character array (that I know of). What variable type should I store the ip in/how should I approach this? Thank you.

possible duplicate of http://stackoverflow.com/questions/6743438/increment-ip-address — yulian, Aug 15 '13 at 16:48

Macattack · Accepted Answer · 2013-08-15T17:01:33.630

A naive implementation (no inet_pton) would user 4 numbers and print them into a char array

#include <stdio.h>

int inc_ip(int * val) {
    if (*val == 255) {
        (*val) = 0;
        return 1;
    }
    else {
        (*val)++;
        return 0;
    }
}   

int main() {
    int ip[4] = {0};
    char buf[16] = {0};

    while (ip[3] < 255) {
        int place = 0;
        while(place < 4 && inc_ip(&ip[place])) {
            place++;
        }
        snprintf(buf, 16, "%d.%d.%d.%d", ip[3],ip[2],ip[1],ip[0]);
        printf("%s\n", buf);
    }
}

*Edit: A new implementation inspired by alk

struct ip_parts {
    uint8_t vals[4];
};

union ip {
    uint32_t val;
    struct ip_parts parts;
};

int main() {
    union ip ip = {0};
    char buf[16] = {0};

    while (ip.parts.vals[3] < 255) {
        ip.val++;
        snprintf(buf, 16, "%d.%d.%d.%d", ip.parts.vals[3],ip.parts.vals[2],
                                        ip.parts.vals[1],ip.parts.vals[0]);
        printf("%s\n", buf);
    }
}

very nice response, [codepade link](http://codepad.org/noapMrLM) of your code — Grijesh Chauhan, Aug 15 '13 at 16:52

score 1 · Answer 2 · edited Jun 16 '14 at 22:18

1

IPV4 addresses are 32bits wide.

Why not take a 32bits wide unsigned integer (uint32_t for example) initialise it to any start value, count it up and convert the result to the dotted string version of the ip-address using the appropriate libc functions?

For further references on the latter please see the man-pages for the inet_XtoY()family of functions.

edited Jun 16 '14 at 22:18

BenMorel

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answered Aug 15 '13 at 16:54

alk

69,737
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Interestingly I was doing the same and what I observed was IPaddress increments from "1.1.1.1" to "2.1.1.1" instead of intended move from "1.1.1.1" to "1.1.1.2". – smRaj May 02 '16 at 13:04
1

@smRaj Network addresses are stored in network-byte-order, which might be different fro the host-bytes-order. Convert the 32bit integer in question using `ntohl()` *before* applying any math on it and `htonl()` afterwards. – alk May 02 '16 at 14:10

score 0 · Answer 3 · answered Aug 15 '13 at 16:40

If you are searching the same subnet 1.1.1. the entire time you can store you last octet as the only int.

int lastoctet = 1;

Loop through it increaseing the lastoctet each time and appending it to your string.

I am not to familiar with C syntax so

//Declare and set int lastoctet = 1
//set ipstring, string ipstring = "1.1.1."
//Loop and each time increase lastoctet
   //ipstring = ipstring & lastoctet.tostring
   //perform actions
   //lastoctet++
//end loop

If you are searching more octets or need t oincrease other digits you can store that octet as a seperate integer and reform your string before or during your looping.

You ... really wouldn't do this. Ever. Network addresses are simply numbers. There are library routines for converted them to and from the textual representations. — Brian Roach, Aug 15 '13 at 16:41

How to increment an IP address in a loop? [C]

3 Answers3

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