A named instance is configured by default to use a dynamic port assignment and such ports can be blocked by a firewall. See http://msdn.microsoft.com/en-us/library/ms345327.aspx

>I have remote users who cannot access a new SQL instance that is running in a three node Windows 2003 server cluster. I and other users can access the SQL instance newname.domainname.ad\instancename. The external users successfully access the current DB server via an external IP address which is not running a SQL instance. However, the users unsuccessfully access the new SQL DB using a fully qualified server name with and without the instance name. Also, the users unsuccessfully access the new SQL DB using the external IP address with and without the instance name.
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>Additionally, I want the network routers to redirect the existing oldname.domainname.ad to redirected to newname.domainname.ad\instancename. Otherwise I will have to go to 600 workstations and update the ODBC connections.