וְהַיַּרְדֵּ֥ן יִגְבֹּל־אֹת֖וֹ לִפְאַת־קֵ֑דְמָה זֹ֡את נַחֲלַת֩ בְּנֵ֨י בִנְיָמִ֧ן לִגְבֽוּלֹתֶ֛יהָ סָבִ֖יב לְמִשְׁפְּחֹתָֽם׃
And the Jordan was to be the border of it on the east side. This was the inheritance of the children of Benjamin, by the borders thereof round about, according to their families.
Read halakhic rulings (Jewish law) on Joshua 18:20: and classical halakhic sources.