The start of the dot com boom I would mark at August 1995 when Netscape’s IPO was the most successful IPO to that date.

But really I mark it with the IETF RFC 1290 “There’s Gold In Them Thar Networks” in 1991.

But yes, the 21st century was defined in 1990’s. Everything since is derivative. I feel sad for those born after everything worth doing had been done already.