Corporate firewall blocks connection to fulfillment server - what port needs to be opened?

Hi,

When users in our network try to open a downloaded .acsm-file they get an error message saying "connection error detected digital editions could not connect to the fulfillment server".

If you take the same computer outside our network you are able to open the file and the book is activated.

What ports on what IP addresses do we need to open to make this work in our corporate network?

Best regards,

Erik