Standalone installation includes following 3 components -
Salesforce.com instance
FTP Attachments® Application Server (Windows Server with IIS)
Storage Server (Where actual files will be stored)
You need to make appropriate firewall settings as following,
Application server should be able to talk to *.salesforce.com on port 443
Application server should be able to talk to *.enzigma.com, *.ftpattachments.com on port 443
Application server should be able to talk to Storage server on appropriate port (e.g. for FTP port 21)
Only port which needs to be open on Application server are 80 and/or 443 (443 is required only if you are using https to access the FTP Attachments®)