File content as of revision 0:8823772081cb:

#include "mbed.h"
#include "UIPEthernet.h"
#include "UIPUdp.h"

// MAC address must be unique within the connected network. Modify as appropriate.
const uint8_t    MY_MAC[6] = { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05 };
// IP address must be unique and compatible with your network.
const IPAddress  MY_IP(192, 168, 1, 181);
const uint16_t   MY_PORT = 7;

Serial           pc(USBTX, USBRX);
UIPEthernet      uIPEthernet(D11, D12, D13, D10);    // mosi, miso, sck, cs
UIPUDP           udp;

/**
 * @brief
 * @note
 * @param
 * @retval
 */
int main(void) {
    uIPEthernet.begin(MY_MAC, MY_IP);
    IPAddress   localIP = uIPEthernet.localIP();
    pc.printf("Local IP = %s\r\n", localIP.toString());
    pc.printf("Initialization ");
    if(udp.begin(MY_PORT))
        pc.printf("succeeded.\r\n");
    else
        pc.printf("failed.\r\n");

    while(1) {
        int size = udp.parsePacket();
        if(size > 0) {
            do {
                char*   msg = (char*)malloc(size + 1);
                int     len = udp.read(msg, size + 1);
                msg[len] = 0;
            } while((size = udp.available()) > 0);

            //finish reading this packet:
            udp.flush();

            int success;
            do {
                //send new packet back to ip/port of client. This also
                //configures the current connection to ignore packets from
                //other clients!
                success = udp.beginPacket(udp.remoteIP(), udp.remotePort());
                if(success)
                    pc.printf("beginPacket: succeeded%\r\n");
                else
                    pc.printf("beginPacket: failed%\r\n");

                //beginPacket fails if remote ethaddr is unknown. In this case an
                //arp-request is send out first and beginPacket succeeds as soon
                //the arp-response is received.
            } while(!success);

            char*   message = "Hello World from mbed";
            success = udp.write((uint8_t*)message, strlen(message));

            if(success)
                pc.printf("bytes written: %d\r\n", success);

            success = udp.endPacket();

            if(success)
                pc.printf("endPacket: succeeded%\r\n");
            else
                pc.printf("endPacket: failed%\r\n");

            udp.stop();

            //restart with new connection to receive packets from other clients
            if(udp.begin(MY_PORT))
                pc.printf("restart connection: succeeded%\r\n");
            else
                pc.printf("restart connection: failed%\r\n");
        }
    }
}