Chat (Telnet) Client

Tutorials > Examples > Ethernet > ChatCllient

In this example, you will use your Ethernet Shield and your Arduino board to make a chat client.

Hardware Required

  • Arduino Ethernet Shield

  • Shield-compatible Arduino board


The Ethernet shield allows you to connect a WizNet Ethernet controller to the Arduino boards via the SPI bus. It uses pins 10, 11, 12, and 13 for the SPI connection to the WizNet. 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.

EthernetShieldF bb

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

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


EthernetShield sch



  Telnet client

 This sketch connects to a a telnet server (

 using an Arduino Wiznet Ethernet shield.  You'll need a telnet server

 to test this with.

 Processing's ChatServer example (part of the network library) works well,

 running on port 10002. It can be found as part of the examples

 in the Processing application, available at


 * Ethernet shield attached to pins 10, 11, 12, 13

 created 14 Sep 2010

 modified 9 Apr 2012

 by Tom Igoe


#include <SPI.h>
#include <Ethernet.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);

// Enter the IP address of the server you're connecting to:

IPAddress server(1, 1, 1, 1);

// Initialize the Ethernet client library
// with the IP address and port of the server
// that you want to connect to (port 23 is default for telnet;
// if you're using Processing's ChatServer, use port 10002):

EthernetClient client;

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 connection:

  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



  while (Ethernet.linkStatus() == LinkOFF) {

    Serial.println("Ethernet cable is not connected.");



  // give the Ethernet shield a second to initialize:



  // if you get a connection, report back via serial:

  if (client.connect(server, 10002)) {


  } else {

    // if you didn't get a connection to the server:

    Serial.println("connection failed");


void loop() {

  // if there are incoming bytes available

  // from the server, read them and print them:

  if (client.available()) {

    char c =;



  // as long as there are bytes in the serial queue,

  // read them and send them out the socket if it's open:

  while (Serial.available() > 0) {

    char inChar =;

    if (client.connected()) {




  // if the server's disconnected, stop the client:

  if (!client.connected()) {




    // do nothing:

    while (true) {




See also