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MAC address

Organizationally Unique Identifier, or OUI

The first three bytes of a MAC address are the Organizationally Unique Identifier, or OUI. These addresses are assigned by the IEEE Registration Authority. You pay about $2000 for a reserved block.

locally administered address

A locally administered MAC address is similar to a LAN IP address (,, and You can make up your own locally administered address and can be sure that it will not collide with any hardware on your network that use a factory burned-in MAC address. Locally administered addresses are useful when creating virtual machines or virtual network interfaces.

The second bit of the first byte of a MAC address determines the type of OUI. If the bit is 0 then it is an OUI globally assigned by the IEEE; if the bit is 1 then it is a locally administered MAC address.

Create a OUI by whatever scheme you like, then logically OR it with 02:00:00:00:00:00, and then logically AND it with fe:ff:ff:ff:ff:ff, and you will have a locally administered address. The first OR pattern sets bit 2 of the first byte; the second AND pattern clears bit 1 of the first byte (unicast, not multicast).

The following MAC address pattern satisfies the OUI requirements:


change MAC address

The following sets eth0 interface to use a locally administered address:

ifconfig eth0 hw ether 02:00:00:00:00:01

or using iproute2 commands you would use the following:

ip link set dev eth0 address 02:00:00:00:00:01

Arduino MAC and IP address hog

This is a DHCP tester made for use with an Arduino and Ethernet Shield.

  MAC and IP address hog
  This sketch repeatedly requests MAC addresses from DHCP.
  A new MAC address is given for each request.
  * Ethernet shield attached to pins 10, 11, 12, 13
  created 2012-07-25
  by Noah Spurrier

#include <SPI.h>
#include <Ethernet.h>

// MAC address template.
// Note that this is a "locally administered MAC address".
// Note that the following is true: "0x4e|0x02 == 0x4e"
byte mac[] = { 0x4e, 0x4f, 0x41, 0x48, 0x0, 0x0 };

void setup() {
  // Open serial port.
  // Wait for serial port to connect. Needed for Leonardo only.
  while (!Serial) ;

  delay (1000); 
  Serial.println("Begin hogging MAC:IP address leases on DHCP.");
  for (mac[4] = 0; mac[4] <= 254; mac[4]++){
    for (mac[5] = 0; mac[5] <= 254; mac[5]++){ 
      Serial.print("MAC: ");
      for (byte thisByte = 0; thisByte < 6; thisByte++) {
        // print the value of each byte of the MAC address:
        Serial.print(mac[thisByte], HEX);
        if (thisByte < 5) Serial.print(":");
      Serial.print(", ");
      Serial.print("IP: ");
      // start new Ethernet connection. See
      if (Ethernet.begin(mac) == 0) {
        Serial.print("ERROR: Failed to get IP address from DHCP.");
      else {
        for (byte thisByte = 0; thisByte < 4; thisByte++) {
          // print the value of each byte of the IP address:
          Serial.print(Ethernet.localIP()[thisByte], DEC);
          if (thisByte < 3) Serial.print("."); 

void loop() {