Cannot connect to a named instance in SQL Server 2012 remotely

Dear all,

            I am using Sql Server 2012. I have created a named Instance called SQLEXPRESS. When I try to connect it by 127.0.0.1\sqlexpress. It is OK. But when I used real IP ( it is on the same machine) to connect , like 121.13.123.124\sqlexpress. It have the following message

          ( Provider. SQL Network Interface error: 26 Error locating Server\Instance Specified).

I have shut Firewall. Set the IP Protocol in configuration Manager and Set the IP2 address to my internal ip. But it still not work. Then

what I done is to installed an Default Instance . Then when I connect to the SQL Server like  121.13.123.124 ( You can see, I do not type the instance name.)  Then it can be connected. No any modification to the Windows Setting and SQL Server setting has been done to make it connected.

But I really hope that I can also connect the named instance by remote IP. So why it is Ok for default instance but not ok for named instance. Did I miss any setting for the named Instance. Please help.

Thanks.

hon123456