I used this to interface with my Arduino via mac Terminal. This bit of code essentially assigns the arduino an IP address using DHCP. DHCP is just a way for routers to automatically generate IP addresses for connecting devices so that users don’t have to set it up manually. I was able to read the assigned IP address, type “telnet 10.0.0.10” or whatever the assigned IP address was into Terminal to connect to the Arduino. if the connection worked, then the arduino responded with hello. anything i typed after that was repeated, or echoed.

 

 

/*
DHCP Chat Server

A simple server that distributes any incoming messages to all
connected clients. To use telnet to your device’s IP address and type.
You can see the client’s input in the serial monitor as well.
Using an Arduino Wiznet Ethernet shield.

THis version attempts to get an IP address using DHCP

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

created 21 May 2011
modified 9 Apr 2012
by Tom Igoe
Based on ChatServer example by David A. Mellis

*/

#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.
// gateway and subnet are optional:
byte mac[] = {
0x90, 0xA2, 0xDA, 0x0F, 0x2A, 0xFB };
IPAddress ip(149,31,195,1);
IPAddress gateway(149,31,195,1);
IPAddress subnet(255, 255, 255, 0);

// telnet defaults to port 23
EthernetServer server(23);
boolean gotAMessage = false; // whether or not you got a message from the client yet

void setup() {
// Open serial communications and wait for port to open:
Serial.begin(9600);
// this check is only needed on the Leonardo:
while (!Serial) {
; // wait for serial port to connect. Needed for Leonardo only
}
// start the Ethernet connection:
Serial.println(“Trying to get an IP address using DHCP”);
if (Ethernet.begin(mac) == 0) {
Serial.println(“Failed to configure Ethernet using DHCP”);
// initialize the ethernet device not using DHCP:
Ethernet.begin(mac, ip, gateway, subnet);
}
// print your local IP address:
Serial.print(“My IP address: “);
ip = Ethernet.localIP();
for (byte thisByte = 0; thisByte < 4; thisByte++) {
// print the value of each byte of the IP address:
Serial.print(ip[thisByte], DEC);
Serial.print(“.”);
}
Serial.println();
// start listening for clients
server.begin();

}

void loop() {
// wait for a new client:
EthernetClient client = server.available();

// when the client sends the first byte, say hello:
if (client) {
if (!gotAMessage) {
Serial.println(“We have a new client”);
client.println(“Hello, client!”);
gotAMessage = true;
}

// read the bytes incoming from the client:
char thisChar = client.read();
// echo the bytes back to the client:
server.write(thisChar);
// echo the bytes to the server as well:
Serial.print(thisChar);
}
}