I'm developing a diagnostic tool on a PC with several Network Interfaces based on multicast/udp. The user can select a NIC, the application creates sockets, binds them to this NIC and adds them to the specific multicast group. The sending of multicast messages works fine. However receiving of messages only succeeds if I bind the sockets to a specific NIC of my PC. It almost looks like as there is a 'default' NIC for receiving multicast messages in Windows which is always the first NIC returned by the GetAdapterInfo function. I monitored the network with Wireshark and discovered that the "IGMP Join Group" message isn't sent from the NIC I bound the socket at, but by this 'default' NIC. If I disable this NIC (or remove the network cable), the next NIC of the list returned by GetAdapterInfo is used for receiving multicast messages. I was successful to change this 'default' NIC by adding an additional entry to the routing table of my PC, but I don't think this is a good solution of the problem. The problem also occurs with the code appended below. The join group messages isn't sent via 192.168.52 but via a different NIC. // socket_tst.cpp : Defines the entry point for the console application. // #include tchar.h #include winsock2.h #include ws2ipdef.h #include IpHlpApi.h #include IpTypes.h #include stdio.h int _tmain(int argc, _TCHAR* argv[]) { WSADATA m_wsaData; SOCKET m_socket; sockaddr_in m_sockAdr; UINT16 m_port = 319; u_long m_interfaceAdr = inet_addr("192.168.1.52"); u_long m_multicastAdr = inet_addr("224.0.0.107"); int returnValue = WSAStartup(MAKEWORD(2,2), &m_wsaData); if (returnValue != S_OK) { return returnValue; } // Create sockets if (INVALID_SOCKET == (m_socket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) ) { return WSAGetLastError(); } int doreuseaddress = TRUE; if (setsockopt(m_socket,SOL_SOCKET,SO_REUSEADDR,(char*) &doreuseaddress,sizeof(doreuseaddress)) == SOCKET_ERROR) { return WSAGetLastError(); } // Configure socket addresses memset(&m_sockAdr,0,sizeof(m_sockAdr)); m_sockAdr.sin_family = AF_INET; m_sockAdr.sin_port = htons(m_port); m_sockAdr.sin_addr.s_addr = m_interfaceAdr; //bind sockets if ( bind( m_socket, (SOCKADDR*) &m_sockAdr, sizeof(m_sockAdr) ) == SOCKET_ERROR ) { return WSAGetLastError(); } // join multicast struct ip_mreq_source imr; memset(&imr,0,sizeof(imr)); imr.imr_multiaddr.s_addr = m_multicastAdr; // address of multicastgroup imr.imr_sourceaddr.s_addr = 0; // sourceaddress (not used) imr.imr_interface.s_addr = m_interfaceAdr; // interface address /* first join multicast group, then registerer selected interface as * multicast sending interface */ if( setsockopt( m_socket ,IPPROTO_IP ,IP_ADD_MEMBERSHIP ,(char*) &imr , sizeof(imr)) == SOCKET_ERROR) { return SOCKET_ERROR; } else { if( setsockopt(m_socket ,IPPROTO_IP ,IP_MULTICAST_IF ,(CHAR*)&imr.imr_interface.s_addr ,sizeof(&imr.imr_interface.s_addr)) == SOCKET_ERROR ) { return SOCKET_ERROR; } } printf("receiving msgs...\n"); while(1) { // get inputbuffer from socket int sock_return = SOCKET_ERROR; sockaddr_in socketAddress; char buffer[1500]; int addressLength = sizeof(socketAddress); sock_return = recvfrom(m_socket, (char*) &buffer, 1500, 0, (SOCKADDR*)&socketAddress, &addressLength ); if( sock_return == SOCKET_ERROR) { int wsa_error = WSAGetLastError(); return wsa_error; } else { printf("got message!\n"); } } return 0; } Thanks four your help!