What url ios trying ping when connect on free wifi hotspot point?

Hi, when i'm connect to free wifi hotspot my iphone trying to open some url and if url is unreachable, iphone show me popup with hotspot start page(billing etc). If give access permanently to this url and iphone success ping it popup won't be shown. On iOS 6 destination url is www.apple.com but its not work on ios7. Please help me and give wifi destination url !