The Book of Revelation actually speaks of eight prophetic kingdoms in all.  John was told that Rome was the sixth kingdom as regards his vision.  By incorporating the prophecies within the Book of Daniel into that supplied by the Book of Revelation, we are able to identify the seven heads of John's beast and therefore the seven different kingdoms referred to. 

The seven heads apparently correspond to the kingdoms of  Babylon, Medo-Persia, Rome, Macedonia, Thrace, Syria, and Egypt.   We learn that  the legs of iron on Nebuchadnezzar's image represented the Roman Empire and the clay in the feet of the image represented the Moslem Empire. 

John's seventh kingdom must follow Rome and so we can easily conclude that the seventh prophetic kingdom of  John's vision is the Moslem Empire.    There is yet an eighth kingdom spoken of by John.  Do you  know who that is?