Genesis 10:21

Unto Shem also, the father of all the children of Eber,
&c.] And for the sake of those Shem is particularly said to be the father of, is this genealogy given, and indeed the whole book of Genesis wrote: Eber was the great-grandson of Shem, and is here spoken of by anticipation, and Shem is called not the father of either of his immediate sons, but of the posterity of this man; because the Hebrews sprung from him in his line, among whom the church of God and the true religion were preserved, and from whom the Messiah was to come, as he did: the word Eber, Jarchi interprets, "beyond the river, Euphrates" or "Tigris", or both, as describing the seat of the posterity of Shem; but as this too much straitens them, since they inhabited on both sides, Dr. Hyde ^F4 has shown that the word used may refer to both, to those beyond these rivers, and to those on this side; see ( Numbers 24:24 )

the brother of Japheth the elder;
he was the brother of Ham too, but he is not mentioned because of the behaviour towards his father, and because of the curse that was upon him and his; but Shem's relation to Japheth is expressed to show that they were alike in their disposition; and it may be to signify, that in times to come their posterity would unite in spiritual things, which has been fulfilled already in part, and will be more fully by the coalition of the Jews, the posterity of Shem, and of the Gentiles, the posterity of Japheth, in the Christian church state: and from hence we learn that Japheth was the eldest of Noah's sons, though some render the words, "the elder brother of Japheth" ^F5; and so make Shem to be the eldest; but as this is contrary to the accents, so to the history: for Noah was five hundred years old when he began to beget sons, ( Genesis 5:32 ) he was six hundred when he went into the ark, ( Genesis 7:11 ) two years after the flood Shem begat Arphaxad, when he was one hundred years old, and Noah six hundred and two, ( Genesis 11:10 ) so that Shem must be born when Noah was five hundred and two years old; and since he begot children, there must be one two years older than Shem, which can be no other than Japheth, since Ham is called his younger son, ( Genesis 9:24 ) .

even to him were [children] born,
who are reckoned as follow.