Technically, it uses whatever port you configure the SQL Server instance to use.  We have a similar setup and use non-standard ports for SQL.  This has saved us numerous times from SQL worms such as Slammer, plus it just makes sense from a security standpoint.  You just need to match the SQL port in the firewall rules and the connection string for DNN.

Jeff

I came here to post the same answer as the previous two.  You shouldn't really use the standard sql server port as it leaves the server vulnerable to attack for sql server exploits.  If you use a non-standard port, then most sql exploits won't affect you, because they won't know what port to look for.  I run all my prod servers with a non-standard port.