Android Question FTP.Initialize Port Number

is the port number always needed?

I am sending a PRN to a printer on the network. Have IP Address.

Am currently using Port 21 (copied from Erel's example)

What are valid ports?
Is a Port Number needed for what I am doing?

How do I know what Port Number the users machine is using?

Is there someway I can retrieve this information?

Thanks

BobVal