My Ethernet shield dont have any MAC address, please tell me how to find the MAC address, I'm stucked! Here is the link from where I bought the Ethernet shield.

2 Answers 2


You can set the Mac address to whatever you want. Many of the example sketches will have one already present, but as long you follow the standard mac format it should be happy (I am using the same shield). In the Examples menu, you should see Ethernet then Webserver where you can find the following example. You can change it a bit, but most of the time I just leave it alone.

byte mac[] = { 
  0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED };
  • "You can set the Mac address to whatever you want." Yes, if you don't care about duplicated addresses, already assigned addresses or even broadcast addresses. To do it right, read a bit about MAC addresses, e.g. starting with Wikipedia about MAC addresses
    – gone
    Aug 12, 2015 at 15:25

Ethernet will work as long as two different machine in the LAN don't have the same MAC address, as @Nathan wrote in his answer.

The MAC address is built by two parts, a prefix which are given to manufacturers of network devices and is called OUI, and a suffix that is the node part that manufacturers can use as they wish.

If you want to build an MAC address yourself, use a Locally administered address. Universally administered and locally administered addresses are distinguished by setting the second-least-significant bit of the most significant byte of the address. That will make this address to not use any OUI prefix, as that bit is zero on OUI, which minimize the risk of two or more devices have the same MAC address in same LAN.

If you get the same MAC addresses you will get very strange results if they happen to be in the same LAN, I tell you. Been there, done that, and it wasn't fun to debug.

Here are a site that looks up MAC addresses (and generate random ones). But if you want to see which manufacturer have that MAC address, you read it from IEEE and their list.

  • so can i use the mac address which is generated by random mac address generator from the link u provided? Feb 19, 2015 at 9:42
  • Yes, you can. But it is still up to you to know that the MAC address is unique for your LAN, because you could theoretically end up with two equal addresses, even though the risk are rather slim. But still slimmer than to use one that are used in examples, like DE:AD:BE:EF:FE:ED. ;-)
    – Anders
    Mar 8, 2015 at 0:44

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