With the net neutrality debate set aside for a bit, Cisco ( CSCO) spies an opportunity in network gatekeeping.

The San Jose, Calif., Internet gearmaker said Thursday that it acquired network security specialist IronPort for $830 million in stock and cash -- the second-biggest sum Cisco has ever paid for a private company.

San Bruno, Calif.-based IronPort develops software and security equipment that helps network operators block unwanted traffic such as spam and system-crippling applications such as spyware. The company has been particularly effective in filtering email by judging the source's reputation.

Using a system called SenderBase, which the company calls "the world's largest email and Web threat detection network and database," IronPort creates a list of undesirable emailers, effectively a blacklist barring entry to the system.

The move comes just a week after regulators approved the merger between AT&T ( T) and BellSouth ( BLS).

Among the heavily debated issues that held up the approval process was the matter of Net neutrality. One camp says big telcos should be required to provide an unfettered Internet. The other camp says phone companies should be allowed to dictate what traffic can pass through and at what quality level.

The Federal Communications Commission made AT&T and BellSouth agree to Net neutrality conditions, but the rules were so loosely framed that some observers called it a victory for big phone.

It may have been just the green light Cisco needed. In the wake of the decision, the networking giant, already eager to sell gatekeeping gear, has now started what looks like a new rush to add more discriminating technology to network security offerings.

Given the recent warming trend in the network gatekeeping sector, RBC analyst Mark Sue saw fit to initiate coverage on Allot Communications ( ALLT) with a buy Thursday. Sue expects to see rising demand for companies that develop products using deep packet inspection.

As the name implies, deep packet inspection helps network operators identify exactly what kind of traffic is moving through the pipes. Sue says this gives telcos and companies running their internal communications networks the ability to prioritize and "optimize their network infrastructure."

The phone companies say the technology can help guarantee different levels of service, giving priority to important time-sensitive info, which presumably can call for first-class pricing.

But industry watchers say discrimination cuts both ways. Outfits such as Net calling giants Skype, a unit of eBay ( EBAY), and Vonage ( VG) flourish on the broadband connections provided by communications titans such as Verizon ( VZ), Comcast ( CMCSA) and AT&T.

The phone and cable companies would obviously like to protect their core business from what they call freeloaders, and suppliers like Cisco, Juniper ( JNPR) and Allot are eager to sell solutions to that problem.

American Technology Research analyst Albert Lin warned in a research note last year that "a delicate and dangerous battle between carriers and major Internet companies will emerge as technology and regulatory rules allow those that own transport networks to begin to prioritize packets for some and 'derate' packets for others."

Since then, the technology has emerged, and now with the FCC's unanimous AT&T/BellSouth merger vote, the regulations seem firmly in the gray.

That's all good news for gearmakers such as Cisco, which aim to please.

"Using the network as a flexible platform to integrate IronPort's technologies," Cisco network security chief Dick Palmer said in a press release Thursday, "Cisco will be able to build new security applications as customers' demands evolve."