diff --git a/shadowsocks-csharp/Encryption/IVEncryptor.cs b/shadowsocks-csharp/Encryption/IVEncryptor.cs
index 8a9d88d2..cf673a66 100755
--- a/shadowsocks-csharp/Encryption/IVEncryptor.cs
+++ b/shadowsocks-csharp/Encryption/IVEncryptor.cs
@@ -106,7 +106,7 @@ namespace Shadowsocks.Encryption
         #region OneTimeAuth
 
         public const int ONETIMEAUTH_FLAG = 0x10;
-        public const int ADDRTYPE_MASK = 0xF;
+        public const int ADDRTYPE_MASK = 0xEF;
 
         public const int ONETIMEAUTH_BYTES = 10;