The twelve regular Sunnahs, concerning the virtue of which it is narrated that the one who offers them regularly will have a house built for him in Paradise, have been explained in the answer to question no. 1048. They are: the two rak’ahs before Fajr, four rak'ahs before Zuhr and two rak'ahs after it, two rak'ahs after Maghrib and two rak'ahs after ‘Isha’.
What you do after the adhaan for Subh and before the obligatory prayer is naafil (supererogatory) and they are the two rak’ahs of Fajr which are called the Sunnah of Fajr or the Sunnah of Subh. They are among the twelve regular Sunnahs, as mentioned above, and your doing them is part of regularly observing this regular Sunnah.
For more information please see the answer to question no. 79345.