וַיְהִ֤י לָהֶם֙ גְּב֣וּל נֶ֔גֶב מִקְצֵ֖ה יָ֣ם הַמֶּ֑לַח מִן־הַלָּשֹׁ֖ן הַפֹּנֶ֥ה נֶֽגְבָּה׃
Rashi on Joshua
And their southern border was. Referring to the southern border of Yehudah.
Rashi on Joshua
From the end of the Salt (Dead) Sea. Referring to the southeastern corner of Eretz Yisroel as stated in Parshas אֵלֶה מַסְעֵי2See Bamidbar 34:12.