A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
jos 21:1EIGHT AND FORTY CITIES GIVEN BY LOT OUT OF THE OTHER TRIBES UNTO THE LEVITES. (Jos 21:1-8)
Then came near the heads of the fathers of the Levites--The most venerable and distinguished members of the three Levitical families, on behalf of their tribe, applied for the special provision that had been promised them to be now awarded (see on Num 35:2). Their inheritance lay within the territory of every tribe. It was assigned in the same place and manner, and by the same commissioners as the other allotments. While the people, knowing the important duties they were to perform, are described (Jos 21:3) as readily conceding this "peculiar" to them, it had most probably been specified and reserved for their use while the distribution of the land was in progress.
jos 21:4the lot came out for the families of the Kohathites--The Levites were divided into Kohathites, Gershonites, and Merarites. Among the former the family of Aaron were exclusively appointed to the priesthood, and all the rest were ranked in the common order of Levites. The first lot was drawn by the Kohathites; and the first of theirs again by the priests, to whom thirteen cities were granted, and ten to the rest of the Kohathites (Jos 21:5); thirteen to the Gershonites (Jos 21:6), and twelve to the Merarites (Jos 21:7).
jos 21:9THE CITIES OF THE PRIESTS. (Jos. 21:9-42)
they gave . . . these cities which are here mentioned by name--It was overruled by the unerring providence of the Divine Lawgiver that the cities of the priests lay within the territories of Judah and Benjamin. This was a provision, the admirable wisdom and propriety of which were fully manifested on the schism that took place in the reign of Rehoboam.
jos 21:41All the cities of the Levites within the possession of the children of Israel were forty and eight cities with their suburbs--This may appear too great a proportion compared with those of the other tribes. But it must be borne in mind that the list given here contains the names of every Levitical city (see on 1Ch. 6:39-66); whereas only those cities of the other tribes are mentioned which lay on the frontier or along the boundary line. Besides, the Levites were not the exclusive inhabitants of those forty-eight cities; for there must have been also a considerable number of people kept there to cultivate the glebe lands and tend the cattle. Still further, the Levitical cities had nothing but "their suburbs round about them" [Jos 21:42]; whereas the other cities in Israel possessed a group of independent villages (see Jos. 17:1-19:51).
jos 21:43GOD GAVE THEM REST. (Jos 21:43-45)
the Lord gave unto Israel all the land which he sware to give unto their fathers--This is a general winding up of the history from the thirteenth chapter, which narrates the occupation of the land by the Israelites. All the promises made, whether to the people or to Joshua (Jos 1:5), had been, or were in the course of being fulfilled; and the recorded experience of the Israelites (Jos 21:45), is a ground of hope and confidence to the people of God in every age, that all other promises made to the Church will, in due time, be accomplished.