This marks the second major acquisition for Citrix this year. In June, it paid about $300 million in cash and stock for high-performance application networking company NetScaler Inc. Before buying NetScaler, Citrix was known for its client/server application delivery technologies, but it's now expanding into the Web-based application marketplace.

Analysts said the deal adds an important component for the NetScaler product line.

Peter Christy, an analyst at Internet Research Group in Los Altos, Calif., said application firewalls are more comprehensive than traditional firewalls because they recognize more levels of attacks. Integrating the Teros firewall with its NetScaler line will yield a more coherent product that's easier for users to install and manage, he said.

Rob Whiteley, an analyst at Forrester Research Inc. in Cambridge, Mass., said that because application firewalls are higher in the IT stack, they can analyze and protect data at a higher level than traditional firewalls.

One problem for Citrix, though, is that it may be unclear who in the IT management chain is responsible for researching and incorporating application firewalls, he said. 'It's just an evolving space, and a lot of clients don't understand its role in their architecture,' Whiteley said.