Symptoms
If you try to connect a clustered named instance of SQL Server through a firewall, and you use only the instance name (for example,SQL_Virtual_Name\Instance_Name) in the connection string, the connection fails and you may receive one of the following error messages:
Error message 1

Specified SQL server not found.
Error message 2

SQL Server does not exist or access denied.If you run a network trace on the server, you can see that the server actually receives the query from the client computer on UDP port 1434 for the virtual server IP address; however, the answer is sent with the physical IP address of the cluster node that is currently running SQL Server.
Resolution
When a client computer connects to a computer that is running a clustered instance SQL Server, if the connection string does not specify the destination TCP port, the client library queries the server on port UDP 1434 to collect information about the instance.
When the server returns the information, the network frame contains the IP address of the physical node instead of the IP address of the virtual server. Depending on the firewall configuration, this network packet may be dropped, and the client may not receive any answer.