There’s an old Wall Street adage: “Buy what you know.” It’s not bad advice, as it points investors toward stocks they can reasonably assess. For the year so far, though, sticking with what you know would have kept most investors clear of the market’s best performing industry, as none of its stocks are household names.

The good news is that it’s not too late to tap into this uptrend. Indeed, given the nature of the business model, it may never technically be too late.

And what’s this hot group? The Internet service, software and support providers. As a group, they’re up by more than +50% this year and still going strong.

The description likely conjures up names like AOL Inc. (NYSE: AOL) or Comcast Corp. (Nasdaq: CMCSA), both of which are well-known players among casual, at-home Web surfers. Those two don’t quite fall into the “Internet service software and support” category, though.

Rather, the group in question includes the likes of F5 Networks (Nasdaq: FFIV), EasyLink Services Intl. (Nasdaq: ESIC) and AboveNet (Nasdaq: ABVT). These technology specialists provide a variety of Internet-related services to organizations with some heavy-duty connectivity needs, solving problems that retail ISPs couldn’t even begin to address — things such as e-commerce, traffic flow management and cloud computing security (ensuring authorized users of Internet-based software and information services are the only ones accessing it). Think of them as the backbone of corporate-level connectivity.

Now, fess up — have you heard of all, or any, of those companies?

There’s no shame if you haven’t, given that most investors and more than a few professional stock-pickers are in the same boat. Yet, considering their performance, it’s a group all investors may want to become more familiar with very soon simply because of the numbers and nature of the business.

Making money in a robust economy is nice, but not particularly challenging. Making money in a lousy economy — like the one we were stewing in for much of 2007 and all of 2008 — is a little more impressive. Making almost as much money in 2008 as you did in 2006 is practically a miracle, but that’s exactly what F5 Networks managed to do despite being smack dab in the middle of a recession. The company brought home $1.02 per share in 2006, $0.90 in 2007, and $0.89 in 2008. Those are results most other companies would have loved to been able to produce at the time, never even mind the fact that F5 posted a record-breaking EPS of $1.68 in 2009.

AboveNet wasn’t up and running in 2007, but since it got the ball rolling in the first quarter of 2008, we’ve seen similar earnings growth trends through the middle of 2010. More of the same is anticipated through 2011.

So what is it about these two companies that allowed them to sail through the recession as if it weren’t happening and then keep on soaring as the recession faded?

F5 and AboveNet, along with EasyLink and many of their peers, have effectively recession-proofed their operations by (1) entrenching themselves in their client companies’ daily operations (to the point of indispensability), and (2) offering a service that draws recurring revenue for “ongoing services rendered.”

And that’s the beauty of the business model. Whereas an auto manufacturer sells one car to one customer without knowing when that buyer may want to buy another vehicle, the Internet software and support providers collect predictable and recurring fees for continually managing an aspect of another corporation’s operation.

F5 (which by the way is the year-to-date leading stock for the group, up nearly +94%) is a prime example of how sweet such a business model can be. While the industry as a whole has seen a general earnings growth trend, F5 Networks has sequentially upped its per-share earnings in each of its past five quarters. It’s also sequentially improved per-share earnings in 10 of the past 11 quarters, the first five of which overlapped with the latter part of the recession.

It’s what the old-schoolers would call a cash cow.

Action to Take –> Value-conscious investors may have a tough time getting on board F5 Networks over AboveNet. The former is sitting on a P/E of more than 60 (though it’s coming down), while the latter boasts a trailing P/E of less than 6.0. In that light alone, AboveNet is the no-brainer choice. When you factor projected growth rates in, though, F5 Networks makes its way back into the mix.

As for other stocks one could use to tap into the Internet service support/software (and recurring-revenue) theme, they’re out there to be sure. Some of them may even offer more attractive recent numbers. The problem is, they all either lack history, are small to the point of being shaky or are foreign equities that are tough to keep good tabs on.

Investors would be better off sticking with either AboveNet or F5. Perhaps a little of both — the best of both worlds — is the solution.