The tech-laden Nasdaq index has hit the 5,000 benchmark for the first time in 15 years. Sure—big, round stock market numbers are purely symbolic, but this one carries with it quite a bit more baggage. To call it a generation's worth wouldn't be unfair.

Last time it ended badly—very badly. The four scariest words in stocks are, of course, "it's different this time," but should investors be wary now?

March of 2000 is the only other month in history the Nasdaq ever laid eyes on 5,000, peaking at 5,135.52. Just five months earlier, the index was valued at half that amount, and by the end of the roller-coaster year it would again be down by half.

But back then, it was more than just smoke and mirrors. It was madness. It was Pets.com and Webvan and $5 trillion of paper wealth just waiting to evaporate. When it rapidly vanished, scores of Silicon Valley office parks were left vacant almost overnight, glutting the market with pool tables, Aeron chairs and kegs of pale ale.

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David Golden, who ran investment banking at Hambrecht & Quist during the bubble years, recalls one particular idiom that encapsulated the mayhem, "If you could fog a mirror, you could go public."

The end result, when the dust finally settled in October 2002, was Nasdaq 1,108.49.

Fast forward to the present and, to use the most dangerous cliché in finance, things are different this time. The new Nasdaq 5,000 is built on the value of every Google search, Facebook status update, Amazon.com purchase, iPhone app and super high-speed device powered by an Intel or Qualcomm processor.

These aren't PowerPoint presentations filled with pie-in-the-sky forecasts. These are some of the biggest franchises in the world, and they've driven the Nasdaq to its current level.

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Are we in another bubble? It's certainly possible. But it's important to see why Nasdaq 5,000 much makes more sense today than it did 15 years ago.

For starters, it's taken more than three years for the index to double this time around versus a few months in the dot-com days. And when was the last time you saw a business like online supermarket HomeGrocer sell shares to the public?

HomeGrocer was readying its IPO when the Nasdaq first topped 5,000, on March 7, 2000. It debuted on March 10, the day the index peaked. The Kirkland, Washington-based company had $21.6 million in annual revenue and a net loss almost four times that large. The valuation: $1.5 billion.