Strange problem with Windows Sockets

[Fabio78s]

Hello,

I have written a simple TCP-IP client to communicate with a microprocessor board with Ethernet
Regardless of what the board does, to open communication it's only necessary to instantiate a socket, write ASCII code 0x15 and wait for the board to reply with ASCII code 0x06
The board comes with a Windows application which the same, then allows to send/receive commands to/from the board

Now, the question is:

if I set the board IP address to a class C one (that is 192.168.100.201) it all works OK either with the board and with my program

However, if I set the IP to a private class A one (e.g. 010.130.001.066) and adjust my NIC settings consequently:
- I'm still able to PING the board
- I'm still able to communicate with the original app
- I'm not able to communicate with my program any more, 'connect' API call returns with timeout error

I have tried to trace the communication with Wireshark. When trying with my program, it shows nothing with the class A ip, imho it means that 'connect' doesn't even try to make a connection

I have also tried with 'Hercules setup utility' (freeware). It has exactly the same behaviour as my program, works only with class C IP

I don't think the cause is related to the uC board, I have the same issue even with the board disconnected -- with class C IP, the program tries to connect, with class A it doesn't

Any clues?

I'm enclosing the source code of my client, however it's nothing different from the classic TCP server you can find on any winsock tutorial

Thank you

---------------------------------------------------------------

#define PORT 6000

//#define IP_TEX "192.168.100.201"  // OK

#define IP_T "010.130.001.066"    // NOT WORKING

#define BUFLEN 512

void f1()
{
    SOCKET SendingSocket;

    SendingSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if(SendingSocket == INVALID_SOCKET)
    {
         printf("Errore : socket() - %ld\n", WSAGetLastError());

         return;
    }

    SOCKADDR_IN ServerAddr;
    ServerAddr.sin_family = AF_INET;
    ServerAddr.sin_port = htons(PORT);
    ServerAddr.sin_addr.s_addr = inet_addr(IP_T);

    if (connect(SendingSocket, (SOCKADDR *) &ServerAddr, sizeof(ServerAddr)))
    {
        printf("Errore: connect() - %ld\n", WSAGetLastError()); // connect hangs here with class A IP, PING still working
        closesocket(SendingSocket);
        return;
    }

    char buf[BUFLEN];
    buf[0] = 21;
    buf[1]=0;

    int BytesSent = send(SendingSocket, buf, strlen(buf), 0);
    if (!BytesSent)
    {
        printf("Errore: send()\n");
        return;
    }

    printf("%d bytes trasmessi\n", BytesSent);
    int rcLen = recv(SendingSocket, buf, BUFLEN, 0);
    printf("%d bytes ricevuti\n", rcLen);
    if (rcLen==1 && buf[0]==0x06) printf("RISPOSTA OK!\n");
}

int main(...)
{
    WSADATA wsaData;
    WSAStartup(MAKEWORD(2,2), &wsaData);
    printf("Winsock DLL OK : %s.\n", wsaData.szSystemStatus);

    f1();
    if(WSACleanup() != 0) printf("Errore: WSACleanup()\n");

    return 0;
}