diff --git a/libraries/Crypto/KeyRing.cpp b/libraries/Crypto/KeyRing.cpp
index 909ff57c..3cc2ef2a 100644
--- a/libraries/Crypto/KeyRing.cpp
+++ b/libraries/Crypto/KeyRing.cpp
@@ -196,6 +196,17 @@ KeyRingClass KeyRing;
  * primary remote device that this device will be communicating with.
  */
 
+/**
+ * \var KeyRingClass::EthernetMACAddress
+ * \brief Identifier for storing a generated MAC address into the other
+ * data section of the key ring.
+ *
+ * When using the Arduino Ethernet library, the MAC address needs to be
+ * provided in the sketch.  This identifier provides a convenient place
+ * to store the MAC address if you want to generate it randomly instead
+ * of specifying a fixed value.
+ */
+
 /** @cond mantis8 */
 
 // 32-bit version of Mantis-8, extracted from the Skinny-C repository:
diff --git a/libraries/Crypto/KeyRing.h b/libraries/Crypto/KeyRing.h
index c2c967da..c360408b 100644
--- a/libraries/Crypto/KeyRing.h
+++ b/libraries/Crypto/KeyRing.h
@@ -65,6 +65,8 @@ public:
     static const uint16_t LocalEd25519Default = 0x4501;     // 'E', 0x01
     static const uint16_t RemoteEd25519Default = 0x6501;    // 'e', 0x01
 
+    static const uint16_t EthernetMACAddress = 0x4D01;      // 'M', 0x01
+
 private:
     static const size_t ChunkSize = 36;