To insure that the clients can connect only specifying the DNS Alias and do not need to specify the SQL Server Name, IP or port, here is a list of the steps needed to implement:

1)Set up the DNS Alias.

2)Determine what ports are going to be used for each SQL Server instance. Registered Ports are those from 1024 through 49151 and Dynamic and/or Private Ports are those from 49152 through 65535. The registered ports number ranges should not be used for named SQL Server instances as a future conflict is possible. Reference http://www.iana.org/assignments/port-numbers

3)Reserve the port numbers so that no other program requests a dynamic port assigned to your SQL server. "How to reserve a range of ephemeral ports on a computer that is running Windows Server 2003 or Windows 2000 Server" http://support.microsoft.com/kb/812873/

4)"Configuring SQL Server Named instances to use a fixed port" http://technet.microsoft.com/en-us/library/ms345327(SQL.90).aspx. This will require a SQL Server restart.

5)Map the DNS aliases to the SQL Server instance name using the SQL Server 2005/2008 Configuration Manager but with SQL Server 2000, use the SQL Server Network Utility. This utility can only be run on the server hosting the SQL Server.