וַיְהִ֗י גְּבוּל֙ בְּנֵ֣י רְאוּבֵ֔ן הַיַּרְדֵּ֖ן וּגְב֑וּל זֹ֣את נַחֲלַ֤ת בְּנֵֽי־רְאוּבֵן֙ לְמִשְׁפְּחֹתָ֔ם הֶעָרִ֖ים וְחַצְרֵיהֶֽן׃ (פ)
And as for the border of the children of Reuben, the Jordan was their border. This was the inheritance of the children of Reuben according to their families, the cities and the villages thereof.
Read kabbalistic (mystical) interpretation of Joshua 13:23: and classical mystical sources.