--- openflow-2009-07-15-2.h	2015-07-23 09:54:15.732479304 -0700
+++ openflow-2009-07-14.h	2015-07-23 09:54:15.736479304 -0700
@@ -246,8 +246,9 @@
 /* Switch features. */
 struct ofp_switch_features {
     struct ofp_header header;
-    uint64_t datapath_id;   /* Datapath unique ID.  Only the lower 48-bits
-                               are meaningful. */
+    uint64_t datapath_id;   /* Datapath unique ID.  The lower 48-bits are for
+                               a MAC address, while the upper 16-bits are
+                               implementer-defined. */
 
     uint32_t n_buffers;     /* Max packets buffered at once. */