Minimum website requirements for ipad to operate

We are starting to deploy some iPad 3's in our production environment. By default, our corporate firewall blocks all websites, except for those that are approved for usage. I connected our new iPad 3 to our wireless and a message came up and said, "cannot connect to itunes store". This is acceptable, since these are production devices used for specific pre-loaded apps that we downloaded and installed via the PC.

However, I was wondering, if anyone knew what specific websites that a iPad goes out and checks in the background without the users knowledge, such as the iTunes store and iCloud for backup?

Thank you for your help.