The Papal residence was not always at the Vatican. During the beginning of the fourth century, Roman Emperor Constantine issued the Edict of Milan, which proclaimed religious toleration throughout the empire. This meant the end of Catholic persecution in Rome. In addition, Constantine gave the palace of Lateran to the Pope, which was converted and enlarged, eventually becoming The Basilica of St. John Lateran (that’s the Basilica di San Giovanni in Laterano if you are going to Italy) and the adjacent Papal palace. The Basilica was dedicated in 324 AD.
Throughout St. John Lateran’s existence, it was attacked and stripped by the Vandals, almost destroyed by an earthquake, and was burnt down twice. After each disaster, it was rebuilt. Eventually, the Basilica was abandoned for a new Papal residence in Avignon (1309 AD). When the Pope’s seat returned to Rome, in the early fifteenth century, St. John Lateran was in ruins. Restoration was again underway. The final restoration that took place, in the latter part of the seventeenth century, changed the appearance considerably, but there are still some surviving elements of that can be found as it stands today. Notably, it retains its original shape, mosaics, and the domed baptismal.
So which St. John was the Basilica named after? Although it was most likely originally dedicated to the Savior, the Basilica was later dedicated to both St. John the Evangelist and St. John the Baptist.
Interesting fact: The Basilica has its own feast day, November 9th, to celebrate its dedication.
For more information, check out: http://www.ewtn.com/library/CHRIST/LATERAN.HTM