From a bit of testing it appears you cannot connect to SQL 2008 R2 using the SQL Server ODBC driver (SQLSRV32.DLL) without specifying the port number.

I can connect successfully using SQL Native Client (SQLNCLI.DLL) and SQL Server Native Cient 10.0 (SQLNCLI10.DLL) without specifying the port.

Looks like all the client connections need to be upgraded to either include the port or use a more current driver (majority of clients are XP machines).

Has anyone had similar experiences and can you advise on the approach taken to resolve?

Thanks