diff --git a/ipv6/dgramopt_posix.go b/ipv6/dgramopt_posix.go
index 46f7f2ff..57143084 100644
--- a/ipv6/dgramopt_posix.go
+++ b/ipv6/dgramopt_posix.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux netbsd openbsd windows solaris
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris windows
 
 package ipv6
 
diff --git a/ipv6/doc.go b/ipv6/doc.go
index 78a779c4..52d2c3dd 100644
--- a/ipv6/doc.go
+++ b/ipv6/doc.go
@@ -239,3 +239,5 @@
 // In the fallback case, ExcludeSourceSpecificGroup and
 // IncludeSourceSpecificGroup may return an error.
 package ipv6 // import "golang.org/x/net/ipv6"
+
+// BUG(mikio): This package is not implemented on NaCl and Plan 9.
diff --git a/ipv6/endpoint.go b/ipv6/endpoint.go
index b988782a..f6a68ab4 100644
--- a/ipv6/endpoint.go
+++ b/ipv6/endpoint.go
@@ -12,6 +12,11 @@ import (
 	"golang.org/x/net/internal/netreflect"
 )
 
+// BUG(mikio): On Windows, the JoinSourceSpecificGroup,
+// LeaveSourceSpecificGroup, ExcludeSourceSpecificGroup and
+// IncludeSourceSpecificGroup methods of PacketConn are not
+// implemented.
+
 // A Conn represents a network endpoint that uses IPv6 transport.
 // It allows to set basic IP-level socket options such as traffic
 // class and hop limit.
diff --git a/ipv6/payload.go b/ipv6/payload.go
index 682ff22f..d9f82251 100644
--- a/ipv6/payload.go
+++ b/ipv6/payload.go
@@ -6,6 +6,9 @@ package ipv6
 
 import "net"
 
+// BUG(mikio): On Windows, the ControlMessage for ReadFrom and WriteTo
+// methods of PacketConn is not implemented.
+
 // A payloadHandler represents the IPv6 datagram payload handler.
 type payloadHandler struct {
 	net.PacketConn