The Jordan forms its boundary on the eastern side. This is the inheritance of the people of Benjamin, according to their clans, boundary by boundary all around.
We have here the lot of the tribe of Benjamin, which Providence cast next to Joseph on the one hand, because Benjamin was own and only brother to Joseph, and was little Benjamin (Psa 68:27), that needed the protection of great Joseph, and yet had a better protector, for the Lord shall cover him all the day long, Deu 33:12.
Commenting on Joshua 18:11-28
And Jordan was the border of it on the east side,.... It had Jordan on the east, Dan on the west, Judah on the south, and Joseph or Ephraim on the north: this was the inheritance of the children of Benjamin, by the coasts thereof round about, according to their families; this is the general description of the limits of this tribe, the particular cities in it follow.
Jos 18:20 The eastern boundary was the Jordan.