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Sending and Receiving String via UDP

In this example, you will use your Ethernet Shield and your Arduino to send and receive text strings via the UDP protocol (Universal Datagram Packet). You'll need another device to send to and from. The Processing sketch included at the end of the code will send to and receive from your Arduino running this example.

Hardware Required


The Ethernet shield allows you to connect a WIZNet Ethernet controller to the Arduino or Genuino boards via the SPI bus. It uses the ICSP header pins and pin 10 as chip select for the SPI connection to the Ethernet controller chip. Later models of the Ethernet shield also have an SD Card on board. Digital pin 4 is used to control the slave select pin on the SD card.

The shield should be connected to a network with an Ethernet cable. You will need to change the network settings in the program to correspond to your network.

Image developed using Fritzing. For more circuit examples, see the Fritzing project page

In the above image, the Arduino or Genuino board would be stacked below the Ethernet shield.



 This sketch receives UDP message strings, prints them to the serial port
 and sends an "acknowledge" string back to the sender

 A Processing sketch is included at the end of file that can be used to send
 and received messages for testing with a computer.

 created 21 Aug 2010
 by Michael Margolis

 This code is in the public domain.

#include <Ethernet.h>
#include <EthernetUdp.h>

// Enter a MAC address and IP address for your controller below.
// The IP address will be dependent on your local network:
byte mac[] = {
  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED
IPAddress ip(192, 168, 1, 177);

unsigned int localPort = 8888;      // local port to listen on

// buffers for receiving and sending data
char packetBuffer[UDP_TX_PACKET_MAX_SIZE];  // buffer to hold incoming packet,
char ReplyBuffer[] = "acknowledged";        // a string to send back

// An EthernetUDP instance to let us send and receive packets over UDP
EthernetUDP Udp;

void setup() {
  // You can use Ethernet.init(pin) to configure the CS pin
  //Ethernet.init(10);  // Most Arduino shields
  //Ethernet.init(5);   // MKR ETH shield
  //Ethernet.init(0);   // Teensy 2.0
  //Ethernet.init(20);  // Teensy++ 2.0
  //Ethernet.init(15);  // ESP8266 with Adafruit Featherwing Ethernet
  //Ethernet.init(33);  // ESP32 with Adafruit Featherwing Ethernet

  // start the Ethernet
  Ethernet.begin(mac, ip);

  // Open serial communications and wait for port to open:
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only

  // Check for Ethernet hardware present
  if (Ethernet.hardwareStatus() == EthernetNoHardware) {
    Serial.println("Ethernet shield was not found.  Sorry, can't run without hardware. :(");
    while (true) {
      delay(1); // do nothing, no point running without Ethernet hardware
  if (Ethernet.linkStatus() == LinkOFF) {
    Serial.println("Ethernet cable is not connected.");

  // start UDP

void loop() {
  // if there's data available, read a packet
  int packetSize = Udp.parsePacket();
  if (packetSize) {
    Serial.print("Received packet of size ");
    Serial.print("From ");
    IPAddress remote = Udp.remoteIP();
    for (int i=0; i < 4; i++) {
      Serial.print(remote[i], DEC);
      if (i < 3) {
    Serial.print(", port ");

    // read the packet into packetBufffer
    Udp.read(packetBuffer, UDP_TX_PACKET_MAX_SIZE);

    // send a reply to the IP address and port that sent us the packet we received
    Udp.beginPacket(Udp.remoteIP(), Udp.remotePort());

  Processing sketch to run with this example

 // Processing UDP example to send and receive string data from Arduino
 // press any key to send the "Hello Arduino" message

 import hypermedia.net.*;

 UDP udp;  // define the UDP object

 void setup() {
 udp = new UDP( this, 6000 );  // create a new datagram connection on port 6000
 //udp.log( true );         // <-- printout the connection activity
 udp.listen( true );           // and wait for incoming message

 void draw()

 void keyPressed() {
 String ip       = ""; // the remote IP address
 int port        = 8888;        // the destination port

 udp.send("Hello World", ip, port );   // the message to send


 void receive( byte[] data ) {          // <-- default handler
 //void receive( byte[] data, String ip, int port ) {   // <-- extended handler

 for(int i=0; i < data.length; i++)

Processing Code

Copy the Processing sketch from the code sample above. When you type any letter in the Processing sketch window, it will send a string to the Arduino via UDP.

See also

Last revision 2018/09/07 by SM