Two things especially are observable in this account of the posterity of Shem: -
I. The description of Shem, Gen 10:21. We have not only his name, Shem, which signifies a name, but two titles to distinguish him by: -
1. He was the father of all the children of Eber. Eber was his great grandson; but why should he be called the father of all his children, rather than of all Arphaxad's, or Salah's, etc.? Probably because Abraham and his seed, God's covenant-people, not only descended from Heber, but from him were called Hebrews; Gen 14:13, Abram the Hebrew. Paul looked upon it as his privilege that he was a Hebrew of the Hebrews, Phi 3:5. Eber himself, we may suppose, was a man eminent for religion in a time of general apostasy, and a great example of piety to his family; and, the holy tongue being commonly called from him the Hebrew, it is probable that he retained it in his family, in the confusion of Babel, as a special token of God's favour to him; and from him the professors of religion were called the children of Eber. Now, when the inspired penman would give Shem an honourable title, he calls him the father of the Hebrews. Though when Moses wrote this, they were a poor despised people, bond-slaves in Egypt, yet, being God's people, it was an honour to a man to be akin to them. As Ham, though he had many sons, is disowned by being called the father of Canaan, on whose seed the curse was entailed (Gen 9:22), so Shem, though he had many sons, is dignified with the title of the father of Eber, on whose seed the blessing was entailed. Note, a family of saints is more truly honourable than a family of nobles, Shem's holy seed than Ham's royal seed, Jacob's twelve patriarchs than Ishmael's twelve princes, Gen 17:20. Goodness is true greatness.
2. He was the brother of Japheth the elder, by which it appears that, though Shem is commonly put first, he was not Noah's first-born, but Japheth was older. But why should this also be put as part of Shem's title and description, that he was the brother of Japheth, since it had been, in effect, said often before? And was he not as much brother to Ham? Probably this was intended to signify the union of the Gentiles with the Jews in the church. The sacred historian had mentioned it as Shem's honour that he was the father of the Hebrews; but, lest Japheth's seed should therefore be looked upon as for ever shut out from the church, he here reminds us that he was the brother of Japheth, not in birth only, but in blessing; for Japheth was to dwell in the tents of Shem. Note, (1.) Those are brethren in the best manner that are so by grace, and that meet in the covenant of God and in the communion of saints. (2.) God, in dispensing his grace, does not go by seniority, but the younger sometimes gets the start of the elder in coming into the church; so the last shall be first and the first last.
II. The reason of the name of Peleg (Gen 10:25): Because in his days (that is, about the time of his birth, when his name was given him), was the earth divided among the children of men that were to inhabit it; either when Noah divided it by an orderly distribution of it, as Joshua divided the land of Canaan by lot, or when, upon their refusal to comply with that division, God, in justice, divided them by the confusion of tongues: whichsoever of these was the occasion, pious Heber saw cause to perpetuate the remembrance of it in the name of his son; and justly may our sons be called by the same name, for in our days, in another sense, is the earth, the church, most wretchedly divided.

Salah - The founder of the people of Susiana.
Eber - See Gen 10:21. The Septuagint add Cainan here, with one hundred and thirty to the chronology.

And Arphaxad begat Salah,.... Or Shelach which signifies "a sending forth"; that is, of waters: it is part of the name of Methuselah, given him by his father, as prophetic of the flood, see Gen 5:21 and Arphaxad, who was born two years after the flood, gives this name to his first born, as commemorative of it: according to some, from him are the Susians (g); and in Susiana is found a city called Sele, by Ptolemy (h); but this seems not to be a sufficient proof:

and Salah begat Eber; from whom, Josephus (i) says, the Jews were called Hebrews from the beginning; and which, perhaps, is as good a derivation of their name as can be given, and seems to be confirmed by Num 24:24 though some derive it from Abraham's passing over the rivers in his way from Chaldea into Syria; but be it so, why might not this name be given to Eber, as prophetic of that passage, or of the passage of his posterity over the Euphrates into Canaan, as well as Eber gave to his son Peleg his name, as a prediction of the division of the earth in his time? the Septuagint version of this text inserts a Cainan between Arphaxad and Salah, but is not to be found in any Hebrew copy, nor in the Samaritan, Syriac, and Arabic versions, nor in Josephus, see Luk 3:36.

(g) Vid. Bochart. Phaleg. l. 2. c. 13. col. 92. (h) Geograph. l. 6. c. 3. (i) Ut supra. (Antiqu. l. 1. c. 6. sect. 4.)

Arphaxad--The settlement of his posterity was in the extensive valley of Shinar, on the Tigris, towards the southern extremity of Mesopotamia, including the country of Eden and the region on the east side of the river.