Downloading file port

I am trying to download the file port. I opened up port 21 to download the tar.gz. It showed connected and logged in. Then I get to "==> PASV ...", and it keeps failing and retrying. It errors out saying it can't connect to the IP/a random port every time, and I have no clue what's going on. Why is it using a random port? What is it trying to do?