The default, unnamed instance always gets port 1433 for TCP. UDP port 1434 is used by the SQL Browser service to allow named instances to be located. In SQL Server 2000 the first instance to be started took this role.
Non-default instances get their own dynamically-allocated port, by default. If necessary, for example to configure a firewall, you can set them explicitly. If you don't want to enable or allow access to SQL Browser, you have to either include the instance's port number in the connection string, or set it up with the Alias tab in cliconfg
(SQL Server Client Network Utility) on each client machine.
For more information see SQL Server Browser Service on MSDN.