diff --git a/specification/appendices/identifier_grammar.rst b/specification/appendices/identifier_grammar.rst
index a0cdf298..b0d4f6f7 100644
--- a/specification/appendices/identifier_grammar.rst
+++ b/specification/appendices/identifier_grammar.rst
@@ -34,7 +34,7 @@ following grammar::
 
     server_name = hostname [ ":" port ]
 
-    port        = *DIGIT
+    port        = 1*5DIGIT
 
     hostname    = IPv4address / "[" IPv6address "]" / dns-name