Aviso FTP (ftp-access.aviso.altimetry.fr) is an authenticated FTP. All users need an account on FTP, whether for NRT  or for DT data, for along-track and gridded products.

Please, subscribe to get access to Ssalto/Duacs products by filling the registration form.

Access restrictions are applied on folders.  Your account gives you an access to a given list of altimetry data. Thus, the folders you're not subscribed to are empty.
Information about products are detailed in the product pages, and a dedicated search products page is available.

Please try to limit simultaneous accesses to one or two, in order to leave bandwidth for others users.

IMPORTANT NOTE

The FTP protocol may not be supported by your web browser anymore.
The alternative is to use a FTP client such as WinSCP, FileZilla, MobaXterm.
For Chrome (up to Chrome 87), it is possible however to re-enable it by following this post from Google support.