How preprocessor at c work?

[alaios]

Dear all,
I am trying to understand how c preprocessor works in c

I have built that simple environment

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>


int main(int argc, char *argv[])
{
	int i;
#ifdef IPV6
    if( ( i = socket( AF_INET6, SOCK_STREAM, 0 ) ) < 0 )
#else
    if( ( i = socket( AF_INET, SOCK_STREAM, 0 ) ) < 0 )
#endif
    perror("error");
    else{
    	printf("socket created %d");

    }

    return 0;
}

and I am trying to see
a) How to set manually IPV6
b) How to make the system set it.

I have heard about a tool that is called autoconf that can do that by it asks for a template for doing that.

Could you please help me with that?

B.R
Alex