diff --git a/include/netinet/arp.h b/include/netinet/arp.h
index b60a0a4f8d8e9..864a1e74b497d 100644
--- a/include/netinet/arp.h
+++ b/include/netinet/arp.h
@@ -72,6 +72,7 @@ struct arpreq
   struct sockaddr arp_netmask;           /* Netmask of protocol address */
   uint8_t         arp_flags;             /* Flags */
   uint8_t         arp_dev[IFNAMSIZ + 1]; /* Device name (zero terminated) */
+  uint8_t         arp_pad[2];            /* Make a four-byte alignment */
 };
 
 /****************************************************************************