Each site gets a fixed and unique IP and you can see this on the control panel as soon as you signup.

As part of the signup and site configuration process you choose an address within our domain ftpstream.com. So if you are the Acme Corporation you might choose acme.ftpstream.com

However you may want to use and address in your domain such as ftp.acme.com. We can't make that address for you as we don't control your domain. However it's a simple matter to make a record in the DNS for your domain associating ftp.acme.com with the IP address of your site.

You will find more details in our support FAQ