Integer to IP Address - C

Question

Hi guys...

I'm preparing for a quiz, and I have a strong suspicion I may be tasked with implementing such a function. Basically, given an IP address in network notation, how can we get that from a 32 bit integer into a string in it's dotted decimal notation (something like 155.247.182.83)...? Obviously we can't be using any type of inet functions either...I'm stumped!

Answer 1

Hint: break up the 32-bit integer to 4 8-bit integers, and print them out.

Something along the lines of this (not compiled, YMMV):

int i = 0xDEADBEEF; // some 32-bit integer
printf("%i.%i.%i.%i",
          (i >> 24) & 0xFF,
          (i >> 16) & 0xFF,
          (i >> 8) & 0xFF,
          i & 0xFF);

Answer 2

Here's a simple method to do it: The (ip >> 8), (ip >> 16) and (ip >> 24) moves the 2nd, 3rd and 4th bytes into the lower order byte, while the & 0xFF isolates the least significant byte at each step.

void print_ip(int ip)
{
    unsigned char bytes[4];
    bytes[0] = ip & 0xFF;
    bytes[1] = (ip >> 8) & 0xFF;
    bytes[2] = (ip >> 16) & 0xFF;
    bytes[3] = (ip >> 24) & 0xFF; 
    printf("%d.%d.%d.%d\n", bytes[3], bytes[2], bytes[1], bytes[0]);        
}

There is an implied bytes[0] = (ip >> 0) & 0xFF; at the first step.

Use snprintf() to print it to a string.

Answer 3

Another approach:

union IP {
    unsigned int ip;
    struct {
      unsigned char d;
      unsigned char c;
      unsigned char b;
      unsigned char a;
    } ip2;
};

...
char  ips[20];
IP ip;
ip.ip = 0xAABBCCDD;

sprintf(ips, "%x.%x.%x.%x", ip.ip2.a, ip.ip2.b, ip.ip2.c, ip.ip2.d);
printf("%s\n", ips);

Answer 4

This is what I would do if passed a string buffer to fill and I knew the buffer was big enough (ie at least 16 characters long):

sprintf(buffer, "%d.%d.%d.%d",
  (ip >> 24) & 0xFF,
  (ip >> 16) & 0xFF,
  (ip >>  8) & 0xFF,
  (ip      ) & 0xFF);

This would be slightly faster than creating a byte array first, and I think it is more readable. I would normally use snprintf, but IP addresses can't be more than 16 characters long including the terminating null.

Alternatively if I was asked for a function returning a char*:

char* IPAddressToString(int ip)
{
  char[] result = new char[16];

  sprintf(result, "%d.%d.%d.%d",
    (ip >> 24) & 0xFF,
    (ip >> 16) & 0xFF,
    (ip >>  8) & 0xFF,
    (ip      ) & 0xFF);

  return result;
}

Answer 5

#include "stdio.h"

void print_ip(int ip) {
   unsigned char bytes[4];
   int i;
   for(i=0; i<4; i++) {
      bytes[i] = (ip >> i*8) & 0xFF;
   }
   printf("%d.%d.%d.%d\n", bytes[3], bytes[2], bytes[1], bytes[0]);
}

int main() {
   int ip = 0xDEADBEEF;
   print_ip(ip);   
}