After the fall of Arnor and then Arthedain, some of the northern Dúnedain became the Rangers of the North while some of the southern Dúnedain became the Rangers of Ithilien.
Some of the northern Dúnedain became the Rangers of the North. This was after the fall of Arnor. This was after then Arthedain. Some of the southern Dúnedain became the Rangers of Ithilien.