I've only commented the parts that are different from the directed_client.
// page04.html,v 1.13 2000/11/27 17:56:43 othman Exp
#include "ace/Log_Msg.h"
#include "ace/SOCK_Dgram_Bcast.h"
#include "ace/INET_Addr.h"
static const u_short PORT = ACE_DEFAULT_SERVER_PORT;
int
main (int argc,char *argv[])
{
  ACE_UNUSED_ARG(argc);
  ACE_UNUSED_ARG(argv);
  ACE_INET_Addr local ((u_short) 0);
  /* Instead of creating the ACE_SOCK_Dgram we created last time,
    we'll create an ACE_SOCK_Dgram_Bcast.  "Bcast" means, of course,
    "Broadcast".  This ACE object is clever enough to go out to the OS
    and find all of the network interfaces.  When you send() on a
    Dgram_Bcast, it will send the datagram out on all of those
    interfaces.  This is quiet handy if you do it on a multi-homed
    host that plays router...  */
  ACE_SOCK_Dgram_Bcast dgram;
  if (dgram.open (local) == -1)
    ACE_ERROR_RETURN ((LM_ERROR,
                       "%p\n",
                       "datagram open"),
                      -1);
  char buf[BUFSIZ];
  sprintf (buf, "Hello World!");
  /* The only other difference between us and the directed client is
    that we don't specify a host to receive the datagram.  Instead, we
    use the magic value "INADDR_BROADCAST".  All hosts are obliged to
    respond to datagrams directed to this address the same as they
    would to datagrams sent to their hostname.
    Remember, the Dgram_Bcast will send a datagram to all interfaces
    on the host.  That's true even if the address is for a specific
    host (and the host address makes sense for the interface).  The
    real power is in using an INADDR_BROADCAST addressed datagram
    against all interfaces.  */
  ACE_INET_Addr remote (PORT,
                        INADDR_BROADCAST);
  ACE_DEBUG ((LM_DEBUG,
              "(%P|%t) Sending (%s) to the server.\n",
              buf));
  if (dgram.send (buf,
                  ACE_OS::strlen (buf) + 1,
                  remote) == -1)
    ACE_ERROR_RETURN ((LM_ERROR,
                       "%p\n",
                       "send"),
                      -1);
  if (dgram.recv (buf,
                  sizeof (buf),
                  remote) == -1)
    ACE_ERROR_RETURN ((LM_ERROR,
                       "%p\n",
                       "recv"),
                      -1);
  ACE_DEBUG ((LM_DEBUG,
              "(%P|%t) The server said:  %s\n",
              buf));
  /* Using the "remote" object instance, find out where the server
    lives.  We could then save this address and use directed datagrams
    to chat with the server for a while.  */
  ACE_DEBUG ((LM_DEBUG,
              "(%P|%t) The server can be found at:  (%s:%d)\n",
              remote.get_host_name(),
              PORT));
  return 0;
}
About that subnet thing:
If you run this client on a host that has multiple network interfaces, the broadcast will go to all of those (sub)networks. What do you do, though, if you need to get past a router? My advice is to write a server that will run on hosts on both sides of your router. When a server on one side of the router receives a broadcast, it would send a directed datagram to it's counterpart on the other side of the router. The counterpart would then re-broadcast the original datagram on that sub-net. Cheap, simple and effective.One final word of warning:
When creating your broadcast datagrams you may see something like this: ACE_SOCK_Dgram_Bcast::mk_broadcast: Broadcast is not enable for this interface.: Unknown error. There are some interfaces (ppp, slip) that don't support broadcast datagrams. That's what you're seeing here.Ok, one more warning:
If you happen to have multiple servers running on your network when you invoke this client, the response could come from any one of them.