Genesis 4:17

And Kain knew his wife, and she conceived and bare Hanok; and he builded a city, and called the name of the city after the name of his son, Hanok.

AND HE BUILDED A CITY, AND CALLED THE NAME OF THE CITY AFTER THE NAME OF HIS SON ENOCH. For at first Cain thought that he would be childless on account of his sin, but after a child was born to him he began to build a city for his son to dwell therein. But because he himself was cursed and his works would not prosper, he called the city “Enoch,” thus proclaiming that he did not build it for himself since he has no city or dwelling place in the land because he was a fugitive and wanderer; rather, the building would be for Enoch, and it is as if Enoch had built it for himself.
Now since it does not say vayiven ir (and he built a city) — [but rather, vayehi boneh ir, which literally means “and he was building a city,”] as it says elsewhere, vayiven (and he builded) Nineveh; And the children of Gad built Dibon — it indicates that he was building the city all his days because his works were cursed. Thus he would build a little with effort and toil, and then move and wander off from that place and return there and build a little more, but he would not prosper in his ways.
The descendants of Cain and his works were recorded in Scripture in order to make known that G-d is long-suffering and that He prolonged the time of his punishment, for he begot children and children’s children, and then it relates how He visited the iniquity of the fathers upon the children,, Verse 7. and his descendants perished. Our Rabbis say that Cain lived many years and that he died in the flood. Thus his hoary head did not go down to the grave in peace, rather he saw his destruction and all his seed with him.
It would appear that Cain’s descendants consisted of only six generations until the flood, while among the descendants of Seth [the third son of Adam] there were an additional two generations before the flood. [It may be that Cain’s descendants consisted of more than six generations before the flood, but Scripture] had no need to relate anything concerning them. It recorded only the names of those who began the building of cities, the grazing of sheep, the art of music, and the skill of working with metals. Scripture also recorded Lamech’s chastisement of his wives in order to tell us that he did beget children but his sons perished before they begot offspring.

וידע קין...ויהי בונה עיר, at the time when a son was born for him he was engaged in building a town to serve as residence for himself and his children in subsequent generations. All of this was east of Eden. This is why he named the town the same as he named his son, i.e. Chanoch.