diff --git a/ryu/ofproto/ofproto_common.py b/ryu/ofproto/ofproto_common.py
index 49d1c12a..cc3820dc 100644
--- a/ryu/ofproto/ofproto_common.py
+++ b/ryu/ofproto/ofproto_common.py
@@ -21,5 +21,7 @@ OFP_HEADER_PACK_STR = '!BBHI'
 OFP_HEADER_SIZE = 8
 assert calcsize(OFP_HEADER_PACK_STR) == OFP_HEADER_SIZE
 
+# note: while IANA assigned port number for OpenFlow is 6653,
+# 6633 is (still) the defacto standard.
 OFP_TCP_PORT = 6633
 OFP_SSL_PORT = 6633