The lot for the tribe of the children of Judah according to their families was to the border of Edom, even to the wilderness of Zin southward, at the uttermost part of the south.
Of course, Judah did not subdue the Philistines who remained powerful enemies of Israel right down to the times of David and Solomon.  As many have pointed out, these boundaries were more ideal than actual, dealing with what God had promised Israel rather than being restricted to what Israel was able to possess.  A comparison with  Numbers 34:3-5 reveals that these are essentially the same boundaries of southern Israel that are mentioned there.