Just before the Flood, we read:
The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown. (Genesis 6:4)
Then, many years later:
And there we saw the Nephilim (the sons of Anak, who come from the Nephilim), and we seemed to ourselves like grasshoppers, and so we seemed to them.” (Numbers 13:33)
Most likely, the word nephilim means “giants” and both verses are simply saying that there were very tall groups of people both before and after the Flood, rather than indicating any ethnic connection between the pre-Flood and post-Flood nephilim.
It’s also worth knowing that it’s at least possible that the Flood wasn’t “universal” (i.e., it didn’t cover the whole earth), which would mean that some nephilim could have been unaffected by it (for more on this, see ‘Did the Flood cover the whole world?‘).