关于IP组播问题

在同一台机运行两个实例，接收正常。
但是在局域网里边就不行了，所谓的局域网就是一台家用路由，连着几台电脑。
在win7与win7（局域网内的另一台电脑），win7与xp（同一台电脑，装在vm里，桥接上网），都接收不了。
怎么才能局域网里接收呀？class multiCast  //发送
    {
        private Socket sock;
        private IPAddress mcIP;
        private int mcPort;
        private IPEndPoint ipep;
        private int ttl = 10;        private Mutex mut;        private string searchString;
        public string SeartchString
        {
            set
            {
                mut.WaitOne();
                searchString = value;
                mut.ReleaseMutex();
            }
            get
            {
                return searchString;
            }
        }
        public multiCast()
        {
            mut = new Mutex();
        }        public multiCast(string IP, string port):this()
        {            searchString = "";
            mcIP = IPAddress.Parse(IP);
            mcPort = Int32.Parse(port);            ipep = new IPEndPoint(mcIP, mcPort);            sock = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);            sock.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.MulticastTimeToLive, ttl);            sock.SetSocketOption(SocketOptionLevel.Socket,SocketOptionName.ReuseAddress, 1);
        }        public void Send()
        {
            
            if (searchString == "")
            {
                return;
            }
            System.Text.UnicodeEncoding encode = new UnicodeEncoding();
            byte[] inputToBeSent = encode.GetBytes(searchString);
            sock.SendTo(inputToBeSent, 0, inputToBeSent.Length, SocketFlags.None, ipep);            SeartchString = "";
        
        }        public void CloseSocket()
        {
            sock.Close();
        }
    }
class multiCastRecv
    {
        private Socket sock;
        private IPAddress mcIP;
        private int mcPort;
        private IPEndPoint receivePoint;
        private EndPoint tempReceivePoint;
        
        private List<string> recvStrings;        public string [] RecvStrings
        {
            get
            {
                string[] strs = new string[recvStrings.Count];
                recvStrings.CopyTo(strs);
                return strs;
            }
        }        private Mutex mut;        public void OpRecvStrs(string op,string recvStr)
        {
            mut.WaitOne();
            if (op == "clear")
            {
                recvStrings.Clear();
            }
            else if (op == "add")
            {
                recvStrings.Add(recvStr);
            }            mut.ReleaseMutex();
        }        public multiCastRecv(string IP, string Port)
        {
            recvStrings = new List<string>();            mut = new Mutex();            mcIP = IPAddress.Parse(IP);
            mcPort = Int32.Parse(
                Port);            sock = new Socket(AddressFamily.InterNetwork,
                                  SocketType.Dgram,
                                  ProtocolType.Udp);            sock.SetSocketOption(SocketOptionLevel.Socket,
                                     SocketOptionName.ReuseAddress, 1);            IPEndPoint ipep = new IPEndPoint(IPAddress.Any, mcPort);            sock.Bind(ipep);            sock.SetSocketOption(SocketOptionLevel.IP,
                                     SocketOptionName.AddMembership,
                                     new MulticastOption(mcIP, IPAddress.Any));            receivePoint = new IPEndPoint(IPAddress.Any, 0);
            tempReceivePoint = (EndPoint)receivePoint;
        }        public void Recieve()
        {
            while (true)
            {
                try
                {                    byte[] recvData = new byte[512];
                    int length = sock.ReceiveFrom(recvData, 0, 512, SocketFlags.None, ref tempReceivePoint);
                    System.Text.UnicodeEncoding encode = new UnicodeEncoding();
                    OpRecvStrs("add", encode.GetString(recvData, 0, length));
                }
                catch
                {
                }
            }
        }        public void CloseSocket()
        {
            sock.SetSocketOption(SocketOptionLevel.IP,
                                    SocketOptionName.DropMembership,
                                     new MulticastOption(mcIP, IPAddress.Any));            sock.Close();
        }
    }