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ICF most commonly refers to Internet Connection Firewall, the built-in firewall in Windows XP that protected computers from unsolicited incoming network traffic. It can also stand for Intersystem Communications Function in IBM mainframe systems, Integrated Circuit Card (though the acronym is ICC), or an Infrastructure Capitalization Fee in a university setting.

Internet Connection Firewall (ICF)

What it is: A security feature in Windows XP that acts as a stateful firewall.

How it works: It blocks unauthorized access by only allowing responses to requests that the computer has already initiated.

Current status: ICF was an earlier version of Windows Firewall, which was enhanced and renamed in a 2004 update.