diff --git a/include/nuttx/net/arp.h b/include/nuttx/net/arp.h
index 52051a52ec..e0fea967dc 100644
--- a/include/nuttx/net/arp.h
+++ b/include/nuttx/net/arp.h
@@ -60,9 +60,13 @@
 
 /* Recognized values of the type bytes in the Ethernet header */
 
-#define UIP_ETHTYPE_ARP 0x0806
-#define UIP_ETHTYPE_IP  0x0800
-#define UIP_ETHTYPE_IP6 0x86dd
+#define UIP_ETHTYPE_ARP 0x0806 /* Address resolution protocol */
+#define UIP_ETHTYPE_IP  0x0800 /* IP protocol */
+#define UIP_ETHTYPE_IP6 0x86dd /* IP protocol version 6 */
+
+/* Size of the Ethernet header */
+
+#define UIP_ETHH_LEN   14      /* Minimum size: 2*6 + 2 */
 
 /****************************************************************************
  * Public Types