Since I was installing a dual instance cluster, I had setup two virtual SQL database names with different IP address and the second instance required a non-default instance name. However, I wanted to access both instances using the virtual name and I found I could do that by setting both of them to listen on port 1433. Now SSMS and other applications can access the second instance using only the SQL server name.
SQLCLUST2012 – default instance name
SQLCLUST2008\SQL2008 – named instance
Can now be accessed simply by their SQL virtual database name
Accessing a clustered SQL Server instance without the instance name.
Note: some problems have been noted with older tools accessing the instance name in this manner.
More details can be found at Ryan McCauley’s blog
Another method for non-clustered servers with multiple instances is to establish an alias for each instance.