The Pasuk states that they came from the east before settling in the valley in Shin'ar. Where, exactly, did they come from?
Rashi: They came from Har ha'Kedem, mentioned earlier (Bereishis 10:30) in connection with the sons of Shem. 1
Ramban (citing the Ibn Ezra): The land of Ararat, where Noach settled after the Flood.
What made them pick the valley in the land of Shin'ar (alias Bavel)?
Rashi: Because it was the only area that was sufficiently large to accommodate them all. 1
Gur Aryeh: The verse states, "and they found a valley," which implies that they had been looking.
QUESTIONS ON RASHI
Rashi writes: "'When they travelled from the east' - where they had been dwelling (10:30)." Ramban asks - That verse refers only to the descendants of Shem; whereas the Tower was built by all the descendants of Noach?
Gur Aryeh: Before the Dispersion, all of the various families of the Bnei Noach lived in close proximity. 1
Ramban himself, however, explains that all of Bnei Noach were centered around Harei Ararat where the Teivah had landed (which is in the east). They now proceeded together to Shin'ar. Verse 10:30 tells us the territory of Shem after the Dispersion.
