Why does it say "Ad Ketzei ha'Yarden"?
Rashi: The width (from south to north) of Yehudah was only opposite Yam ha'Melach, i.e. until the point where the Yarden enters it.
Where is the northern border of Yehudah?
Rashi: It begins [in the east] where the Yarden enters Yam ha'Melach, and goes west to Beis Chaglah