Ethernet Sheild; MAC Address Confusion


I’ve just purchased a seedstudio Ethernet shield 1.0 from my local radio shack to plug into my Arduino Uno. The only sticker and coordinated print on the board is 12A12, which may be the boards MAC address? I am new to binary:decimal:hexadecimal conversions in coding and such, so despite my research I’m having trouble understanding how to convert this or if I should use this or convert my computer’s ethernet MAC address into the intro code’s format? My computer says the Ethernet’s MAC address is c8, bc, c8 ,91, 49, 52. What are these various formats so I can research conversions in the future?

The test code I’m working with is as follows:
Web Server

A simple web server that shows the value of the analog input pins.
using an Arduino Wiznet Ethernet shield.


  • Ethernet shield attached to pins 10, 11, 12, 13
  • Analog inputs attached to pins A0 through A5 (optional)

created 18 Dec 2009
by David A. Mellis
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);

// Initialize the Ethernet server library
// with the IP address and port you want to use
// (port 80 is default for HTTP):
EthernetServer server(80);

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

// start the Ethernet connection and the server:
Ethernet.begin(mac, ip);
Serial.print("server is at ");

void loop() {
// listen for incoming clients
EthernetClient client = server.available();
if (client) {
Serial.println(“new client”);
// an http request ends with a blank line
boolean currentLineIsBlank = true;
while (client.connected()) {
if (client.available()) {
char c =;
// if you’ve gotten to the end of the line (received a newline
// character) and the line is blank, the http request has ended,
// so you can send a reply
if (c == ‘\n’ && currentLineIsBlank) {
// send a standard http response header
client.println(“HTTP/1.1 200 OK”);
client.println(“Content-Type: text/html”);
client.println(“Connection: close”); // the connection will be closed after completion of the response
client.println(“Refresh: 5”); // refresh the page automatically every 5 sec
// output the value of each analog input pin
for (int analogChannel = 0; analogChannel < 6; analogChannel++) {
int sensorReading = analogRead(analogChannel);
client.print(“analog input “);
client.print(” is “);
if (c == ‘\n’) {
// you’re starting a new line
currentLineIsBlank = true;
else if (c != ‘\r’) {
// you’ve gotten a character on the current line
currentLineIsBlank = false;
// give the web browser time to receive the data
// close the connection:
Serial.println(“client disonnected”);

The format for the MAC address, as I’m sure you can see is:

byte mac = {
0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };

I have changed the IP address to match my Ethernet IP address but when I typed that IP address into my chromium browser bar it came up network unavailable, everything else I’ve tried says web page is not available. I’m sure I am under informed. I’m not sure in which direction. Please help, and thank you in advance for your time and attention.

The newer ethernet shields come with a label for a recommended mac address, but the one in the examples is ok too. Just insure there are no duplicate mac addresses or ip addresses on the same localnet.

BTW, if you didn't notice

byte mac[] = {   0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };

spells DEAD BEEF FEED. It is the most common mac address used in the examples.

Oh funny. I had not noticed. So what do you call the 0xDE in a 6 string? Is that hex? Can I just make up my own if I run into trouble with the ole DEAD BEEF FEED?

Yes you can. Many times I change the first hex number to a zero and the last number incremented by one.

byte mac[] = {   0x00, 0xAD, 0xBE, 0xEF, 0xFE, 0xEF };