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Silicon Valley is starting to feel a little less exuberant these days.

Some big start-ups are having trouble raising money. Others are delaying plans to go public. Hot new technologies like Bitcoin have been struggling.With all the nervous chatter out there — Where is this going? What’s next? — I called on Marc Andreessen, who has lived through a bubble or two (or three).

Mr. Andreessen, a prominent venture capitalist, has been at the epicenter of the technology business here for two decades. In the early 1990s, he was a co-founder of one of the first major web browsers, Netscape. He went on to start Ning, an early social site. Now, in addition to being voluble on Twitter, he sits on the board of Facebook, eBay and Hewlett-Packard.

I got 28 minutes with him. But Mr. Andreessen speaks so quickly, it felt like an hour and a half.

Saying “bubble” to a venture capitalist around here is like saying “homework” to a teenager. They roll their eyes and try to shoo you away. So rather than ask if we are in the middle of another — ahem — bubble, I asked Mr. Andreessen why some young companies can’t seem to raise all the money they want.

His answer: conditions ebb and flow. And right now, they are ebbing for some companies. But for others, capital is still available.

“We have not seen any pullback so far on private investment,” Mr. Andreessen said. “Actually, what we’re seeing is more of the opposite right now. We’re seeing a lot more institutional money — in particular from the hedge fund world — crossing over” into tech investing.

But what about companies that can’t find their way to the stock market, like Box and Square?

Mr. Andreessen pointed to 1997 and 1998, when the market for initial public stock offerings was briefly derailed by the Asian financial crisis. Before long, investor interest revved up again, and the Nasdaq stock market was flying high. (A few years later, of course, it came crashing down.)

So if a start-up can’t go public just now, it’s no big deal. “The nature of the public market is that it is manic depressive,” Mr. Andreessen said. “It gets excited, it gets depressed.”

Speaking of feeling down, I asked Mr. Andreessen a question I often hear from people outside Silicon Valley: Why can’t other places build their own valley-style tech hubs? People in cities ranging from Dublin to Berlin to New York often ask what they are doing wrong.

Mr. Andreessen said new valleys will eventually emerge. But they won’t be Silicon Valley copycats.

Over the past couple of years, venture firms have invested in start-ups in Los Angeles, New York, Chicago and all over China. Los Angeles, for example, is home to Snapchat, Tinder, Whisper, Oculus VR and Beats, some of the big tech stories of the year. Mr. Andreessen said another hot place is Atlanta, the home of Georgia Tech.

But he offers a caveat.

“My personal view is that Silicon Valley will continue to take a disproportionate share of the No. 1 positions in great new markets, and I think that’s just a reflection that the fact that the valley works as well as it does,” Mr. Andreessen said.

There is a caveat to his caveat.

In Mr. Andreessen’s view, there shouldn’t be 50 Silicon Valleys. Instead, there should be 50 different kinds of Silicon Valley. For example, there could be Biotech Valley, a Stem Cell Valley, a 3-D Printing Valley or a Drone Valley. As he noted, there are huge regulatory hurdles in many of these fields. If a city wanted to spur innovation around drones, for instance, it might have to remove any local legal barriers to flying unmanned aircraft.

So what kind of valley would Marc Andreessen build? He is a big believer in digital currencies like Bitcoin, as well as in virtual reality. He says enterprises like Bitcoin will change the world on the scale that web browsers did. And before long, virtual reality will be, well, reality.

Don’t believe him? Mr. Andreessen recalled similar skepticism in May 1994 when he was showing media and telecommunications executives this bizarre development called a web browser.

“They pretty much told us we were out of minds and to go away,” he said. Five years later, Netscape was sold to AOL for $4.2 billion.

Finally, I couldn’t end my discussion without asking him about Twitter. Over the past several months Mr. Andreessen has become a mantle on the social network, known for his long stream of tweets that have become known as the “Marc Andreessen Tweetstorm.”

So Mr. Andreessen, why the long tweets?

“My view is that 140 characters doesn’t make sense anymore,” he said. “If it was me, I’d immediately raise the limit to 300 or 400 characters.”

But if that doesn’t happen, he’s incredibly bullish on the service just the way it is. “The counterargument is that there is magic to the limitation,” he said, laying out a number of virtues around the limited character lengths. “If Twitter doesn’t adapt, I would not view them as being stupid. I just hope they change it because it would be a lot better for me.”