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.