#include <Arduino.h>
#include <Ethernet2.h>    // For Wiznet W5500 chip

static byte MAC[] =       {0x74,0x69,0x69,0xef,0xef,0xef};
static EthernetServer     PORT23(23);  /* Listen on telnet port 23 for inbound connections */

void loop()
{
  EthernetClient port23 = PORT23.available();
  if (port23 && port23.connected()) {
    Serial.println("port23 Connect");
    while (port23) {
      if (!port23.connected()) {
        port23.stop();
      } else {
        while (Serial.available())
          port23.write(Serial.read());
      }
      int len = port23.available();
      if (len) {
        byte buf[128];
        if (len > sizeof(buf))
          len = sizeof(buf);
        len = port23.read(buf, len);
        Serial.write(buf, len);
      }
    }
    Serial.println("port23 Disconnect");
  }
}

void setup()
{
  Serial.begin(115200);
  delay(1000);
  Serial.println("Setup()");
  Ethernet.begin(MAC);
  Serial.println("DHCP done.");
  delay(200); // Necessary??
  Serial.print("Local IP is ");
  Serial.println(Ethernet.localIP());
  PORT23.begin();  // Start listening for inbound connections
  Serial.println("Listening on TCP port 23");
}