Example of getting unique mac address using Ethernet Featherwing

example.ino

#import "macAddress.h"

void setup () {
  byte mac[6];
  getUniqueMacAddress(mac);
  
  Serial.begin(115200);

  Serial.print("mac: "); 
  for (uint8_t i = 0; i < sizeof(mac); i++) {
    Serial.print(mac[i], 16); Serial.print(" ");
  }
}

void loop () {
  
}

macAddress.h

#include "Arduino.h"

#ifndef macAddress_H
#define macAddress_H

/**
 * Gets a unique mac address at runtime
 *  
 * The Ethernet chip doesn't come with a mac address set on it so you have to supply one.
 * This line ensures that the mac address is unique among devices on the lan – at least among
 * RSG devices using ESP8266 – by using the unique chip id that the ESP8266 does come with (not
 * to be confused with the ethernet featherwing chip), and using that as the unique last four
 * bytes of the mac address
 * 
 * @param mac 6 byte mac address container to be overwritten
 * @author Andrew Blair 
 */
void getUniqueMacAddress(uint8_t* mac) {
  union ChipID {
     byte bytes[4];
     uint32_t value;
  };
  
  union ChipID chipId;
  chipId.value = ESP.getChipId();
  
  // 6-byte mac address
  mac[0] = 0xDE; // arbitrary
  mac[1] = 0xAD; // arbitrary
  mac[2] = chipId.bytes[3]; // 4th byte of uint32_t...
  mac[3] = chipId.bytes[2];
  mac[4] = chipId.bytes[1];
  mac[5] = chipId.bytes[0];
}

#endif //macAddress_H