What is "Nachalah Lo Yih'yeh ... " referring to?
Rashi #1 and Targum Yonasan: It refers to a regular portion 1 in Eretz Yisrael.
Rashi #2 (citing the Sifri): "ve'NachalahLo Yihyeh Lo" This is Nachalas She'ar - the inheritance of other nations, with reference to the Keini Kenizi and Kadmoni; "be'Kerev Echav" - is the inheritance of five, with reference to the five Cana'ani nations of the West Bank of the Yarden, and the lands of Sichon and Og in the East Bank of the River Yarden, make up the other two Cana'ani nations. 2
Rashi (citing Rabeinu K'lonimus's version of the Sifri): "ve'Nachalah Lo Yih'yeh lo" - This is Nachalas Chamishah, with reference to the five tribes, who received their inheritance from Moshe 3 and Yehoshua; 4 "be'Kerev Echav" - is the inheritance of the other seven tribes, who distributed the inheritance themselves after Yehoshua died.
Ramban: Refer to 18:2:1:2. The five and the seven refer respectively, to the five Cana'ani nations - the Cana'ani, the Chiti, the Emori, the Chivi and the Yevusi - whose land flowed with milk and honey 5 - the major inheritance of Eretz Yisrael, and the two nations to the P'rizi and the Girgashi, whose land did not.
Targum Yonasan: Of fields and vineyards.
Rashi: Incorporating the Emori and the Cana'ani. (It is unclear however, where we find that the Cana'ani resided in Eiver ha'Yarden?)
Rashi: Reuven, Gad and half of Menasheh.
Rashi: Yehudah, Efrayim and the other half of Menasheh.
When did Hashem tell the Kohanim this?
Rashi: In Korach, Bamidbar, 18:20, when He said to them "In their land you shall not inherit ... I am your portion!"