Ethernet hwaddr and EEPROM storage with Arduino

Posted by mitch on October 31, 2012
hardware, projects, software

There are lots of examples of how to use the Ethernet Wiznet chips with Arduino, whether as Ethernet shields or as Ethernet Arduinos on a single board. Unfortunately, most of these examples hard-code the hardware (MAC) address, which can make things painful if you’re building more than one device and running them on the same network.

The code snippet below is a more convenient approach. You can setup a prefix (DEADBEEF in the example below) for the hardware address and the last two bytes are set randomly on first boot. The hardware address is stored in EEPROM (7 bytes are needed, 1 for a flag indicating that the next 6 bytes are properly populated).

The bytes->String conversion below is a bit ugly but I didn’t think I wanted the overhead of sprint in this. It is probably not worth the trade off. (0x30 is ‘0’ and 0x39 is ‘9’. Adding 0x07 skips over some ASCII characters to ‘A’.)

Some serious caveats: There’s only two bytes of randomness here. You might want more. Ideally you would have a manufacturing process, but if you’re just building six devices, who cares? Clearly you would never use this approach in a production environment, but it’s easier than changing the firmware for every device in a hobby environment. You could also use a separate program to write the EEPROM hardware address and keep this “manufacturing junk” out of your main firmware. These issues aside, my main requirement is convenience: I want to be able to burn a single image onto a new board and be up and running immediately without having to remember other steps. Convenience influences repeatability.

#include <Ethernet.h>
#include <EEPROM.h>

// This is a template address; the last two bytes will be randomly
// generated on the first boot and filled in.  On later boots, the
// bytes are pulled from EEPROM.
byte NETWORK_HW_ADDRESS[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0x00, 0x00};

// These are commented out so that this code will not compile
// without the reader modifying these lines.  If you are using
// EEPROM code in your program already, you need to put the
// network address somewhere that doesn't collide with existing use.

// Call this from your setup routine (see below)
initEthernetHardwareAddress() {
    int eeprom_flag =;  
    int i;
    Serial.print("EEPROM flag is " + String(eeprom_flag));
    if (eeprom_flag != 0xCC) {
        NETWORK_HW_ADDRESS[4] = random(255);
        NETWORK_HW_ADDRESS[5] = random(255);  
        // write it out.
        Serial.println("Writing generated hwaddr to EEPROM...");
        for (i = 0; i < 6; i++) {
            EEPROM.write(EEPROM_HWADDR_START_ADDR + i + 1,

    } else {
        Serial.print("Reading network hwaddr from EEPROM...");
        for (i = 0; i < 6; i++) {
            NETWORK_HW_ADDRESS[i] =
       + i + 1);
    char hw_string[13];
    hw_string[12] = '\0';
    for (i = 0; i < 6; i++) {
        int j = i * 2;
        int the_byte    = NETWORK_HW_ADDRESS[i];
        int first_part  = (the_byte & 0xf0) >> 4;
        int second_part = (the_byte & 0x0f);
        first_part  += 0x30;
        second_part += 0x30;
        if (first_part > 0x39) {
            first_part += 0x07;
        if (second_part > 0x39) {
            second_part += 0x07;
        hw_string[j] = first_part;
        hw_string[j + 1] = second_part;

    NETWORK_HW_ADDRESS_STRING = String(hw_string);


setup() {
    // first call the usual Serial.begin and so forth...

    // setup the Ethernet hwaddr before you start using networking

    int dhcp_worked = Ethernet.begin(NETWORK_HW_ADDRESS);

    // ...