Albert Barnes Commentary Numbers 21:24

"And Israel smote him with the edge of the sword, and possessed his land from the Arnon unto the Jabbok, even unto the children of Ammon; for the border of the children of Ammon was strong." — Numbers 21:24 (ASV)

Jabbok (now Wady Zerka: compare Genesis 32:22) runs eastward under Rabbah of the children of Ammon, then westward, and reaches the Jordan, 45 miles north of the Arnon. It was between Rabbah and Gerasa that it formed the Ammonite boundary.