Increment IP address

Question

In that program I want to increment IP address. And I see output like that:

125.23.45.67
126.23.45.67
127.23.45.67 
128.23.45.67
129.23.45.67
130.23.45.67
131.23.45.67
132.23.45.67
133.23.45.67
134.23.45.67

But I want to see output like this:

124.23.45.67
124.23.45.68
124.23.45.68 
124.23.45.70
124.23.45.71
124.23.45.72
124.23.45.73
124.23.45.74
124.23.45.75
124.23.45.76

Here is program code:

#include <stdlib.h>
#include <stdio.h>
#include <iostream>
using namespace std;
#include "winsock2.h"
#pragma comment(lib,"wsock32.lib")

void main()
{
in_addr adr1;
in_addr adr2;
int i;

adr1.s_addr=inet_addr("124.23.45.67");
adr2.s_addr=inet_addr("as.34.34.56");
if (adr1.s_addr!=INADDR_NONE)
    cout << " adr1 correct" << endl;
else
    cout << " adr1 incorect " << endl;

if (adr2.s_addr!=INADDR_NONE)
    cout << " adr2 correct" << endl;
else
    cout << " adr2 incorect" << endl;

cout << inet_ntoa(adr1) << endl;
cout << inet_ntoa(adr2) << endl;

for (i=0;i<10;i++)
{
    adr1.s_addr ++;
    cout << inet_ntoa(adr1) << endl;
}
}

Answer 1

Big endian and little endian gets another one! Use htonl and ntohl to convert back and forth.

for (i=0;i<10;i++)
{
    adr1.s_addr  = htonl(ntohl(adr1.s_addr) + 1);

    cout << inet_ntoa(adr1) << endl;
}

Answer 2

To increment an IP address you will need to break up the in_addr object into 4 int objects (a short int will also do) and increment the 4th one until it hits 256, and then reset it to 1 and increment the 3rd one, etc. You shouldn't be using ++ on the in_addr object directly.

EDIT: Okay, so you can properly increment it if you reverse the byte order. I personally wouldn't do it that way. Especially if all you're doing is outputting IP strings and not using them as an in_addr elsewhere in code.

Answer 3

Instead of using adr1.s_addr:

adr1.s_addr=inet_addr("124.23.45.67");
adr2.s_addr=inet_addr("as.34.34.56"); 

Use this:

u_long addr1=inet_addr("124.23.45.67");

And increment addr1, i.e. addr1++ the last octet gets incremented.

Or follow this formula:

if IP is A.B.C.D then u_long addr = A + 256*B + 256*256*C + 256*256*256*D