A类互联网网址： 7位网络号          24位主机号 
              01111111.11111111.11111111.11111111 --->01111111 = 127 
               0..127  0..255   0..255   0..255 
               0.0.0.0 -- 127.255.255.255 
B类互联网网址： 14位网络号          16位主机号 
              10111111.11111111.11111111.11111111 --->10111111 = 191 
               128..191  0..255   0..255   0..255 
               128.0.0.0 -- 191.255.255.255 
C类互联网网址： 21位网络号          8位主机号 
              11011111.11111111.11111111.11111111 --->11011111 = 223 
               192..223  0..255   0..255   0..255 
               192.0.0.0 -- 223.255.255.255 
多播地址： 224.0.0.0 -- 255.255.255.255

const  
  MCASTADDR     = ‘234.5.6.7’;  
  MCASTPORT     = 25000;  
  LOCAL_PORT    = 5555;  
  WM_SOCKET     = WM_USER + 55;  
type  
  TForm1 = class(TForm)  
    Button1: TButton;  
    Edit1: TEdit;  
    ListBox1: TListBox;  
    CheckBox1: TCheckBox;  
    procedure FormCreate(Sender: TObject);      
    procedure Button1Click(Sender: TObject);  
    procedure FormClose(Sender: TObject; var Action: TCloseAction);  
  private  
    { Private declarations }  
    procedure WMSocket(var Msg: TMessage); message WM_SOCKET;  
  public  
    { Public declarations }  
  end;  
var  
  Form1: TForm1;  
implementation  
{$R *.dfm}  
procedure ErrMsg( msg: string );  
begin  
  MessageBox( 0, PChar(msg), ‘Ping Program Error’, MB_ICONERROR );  
end;  
procedure TForm1.WMSocket(var Msg: TMessage);  
var  
  buf:array[0..255]of char;  
  remote:TSockaddr;  
  len:integer;   p:string;  
begin  
  case WSAGetSelectEvent( Msg.LParam ) of  
    FD_READ :  
    begin  
      recvfrom(msg.WParam, buf[0], 255,0, remote, len );  
      getpeername(  msg.WParam, remote, len );  
      p :=  inet_ntoa(remote.sin_addr) + ‘Port: ’ + inttostr(ntohs(remote.sin_port));  
      listbox1.Items.Add(p + buf);  
    end;  
  end;  
end;  
procedure TForm1.FormCreate(Sender: TObject);  
var  
  wsa : TWSAData;  
begin  
  if WSAStartup( $0202, wsa ) <> 0 then  
     ErrMsg( ‘Windows socket is not responed.’);  
  listbox1.Font.Name := ‘Courier New’;  
  listbox1.Font.Size := 9;  
end;  
procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);  
begin  
  if WSACleanup <> 0 then  
     ErrMsg( ‘Windows socket can not be closed.’ );  
end;  
procedure TForm1.Button1Click(Sender: TObject);  
var  
  sock : TSocket;  
  local,  
  remote    : TSockAddr;  
  bufsend  : Array[0..31]of char;  
  ret,i      : s32;  
  ttl, opt : s32;  
  bSender :bool;  
  mcast   :ip_mreq;  
begin  
  bSender := CheckBox1.Checked;  
  sock := socket( AF_INET, SOCK_DGRAM, 0 );  
  if sock = INVALID_SOCKET then  
  begin  
    ErrMsg( ‘socket Failed: ’ + IntToStr(WSAGetLastError) );  
    exit;  
  end;  
  local.sin_family := AF_INET;  
  local.sin_port := htons(u16(MCASTPort));  
  local.sin_addr.S_addr := inet_addr(‘192.168.1.12’);  
  //  
  if bind( sock, @local, SizeOf(local) )= SOCKET_ERROR then  
  begin  
    ErrMsg( ‘bind Failed: ’ + IntToStr(WSAGetLastError) );  
    closesocket(sock);  
    exit;  
  end;  
  if WSAAsyncSelect( sock, Handle, WM_SOCKET, FD_READ ) = socket_error then  
  begin  
    ErrMsg( ‘WSAAsyncSelect Failed: ’ + IntToStr(WSAGetLastError) );  
    closesocket(sock);  
    exit;  
  end;  
  remote.sin_family := AF_INET;  
  remote.sin_port := htons( MCASTPort );  
  remote.sin_addr.S_addr := inet_addr(MCASTADDR);  
  mcast.imr_multiaddr.S_addr := remote.sin_addr.S_addr;  
  mcast.imr_interface.S_addr := local.sin_addr.S_addr;  
  if setsockopt( sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, @mcast, sizeof(mcast) )=SOCKET_ERROR then  
  begin  
    ErrMsg( ‘setsockopt Failed: ’ + IntToStr(WSAGetLastError) );  
    closesocket(sock);  
    exit;  
  end;  
  Opt := 8;  
  if setsockopt( sock, IPPROTO_IP, IP_MULTICAST_TTL, @opt, sizeof(opt) )=SOCKET_ERROR then  
  begin  
    ErrMsg( ‘setsockopt Failed: ’ + IntToStr(WSAGetLastError) );  
    closesocket(sock);  
    exit;  
  end;  
  //要发送的UDP数据
  FillChar( bufSend, 32, ‘C’ );  
  if bSender then  for i:=0 to 3 do  
  begin  
    ret :=sendto( sock, bufsend[0], 32, 0, remote, sizeof(remote));  
    if ret =socket_error then  
    begin  
      ErrMsg( ‘sendto Failed: ’ + IntToStr(WSAGetLastError) );  
      //closesocket(sock);  
      break;  
    end;  
  end;  
  //closesocket( sock );  
 // closesocket(sockm);  
end;