F

failover

A configuration that allows a secondary machine to take over in the event of a stoppage in the first machine, thus allowing normal use to return or continue.
See also high availability.

fail-shut mode

A condition in which a firewall blocks all incoming and outgoing network traffic in the event of a firewall failure. This is the opposite of fail-open mode, in which a firewall crash opens all traffic in both directions.

fast Ethernet

An Ethernet networking system that transmits data at 100 million bits per second (Mbps), ten times the speed of an earlier Ethernet standard. Derived from the Ethernet 802.3 standard, it is also known as 100Base-T.

file server

A dedicated network computer that stores data files so that other computers can share access to them.
See also client/server.

filtering process

Deciding whether a packet should be allowed or denied, depending on what is contained in its header or its contents, based on user-defined policies.

fingerprint

A unique identifier for a key that is obtained by hashing specific portions of the key data.
See one-way hash function.

file extension

Under Windows, a period and up to three characters at the end of a file name. The extension can help identify the type of file, and often helps a computer know what to do with the file. For example, if a file is named glossary.exe, the file extension is ".exe." The .exe tells a Windows computer that the glossary file is executable.

filters

Small, fast programs in a firewall that examine packets as they arrive at the firewall, and route or reject the packets based on user-definable rules.

firewall

Software or hardware components that restrict access between a protected network and the Internet, or between other sets of networks, to block unwanted use or abuse.

forward DNS lookup

See DNS lookup

FQDN (Fully Qualified Domain Name)

A fully qualified domain name consists of a host and domain name, including a top-level domain such as .com, .net, .gov, .edu, etc. For example, www.domain.com is a fully qualified domain name. www is the local host, domain is the second-level domain, and .com is the top level domain.

FTP (File Transfer Protocol)

The most common protocol for copying files over the Internet.
See also active mode FTP.

Fully Qualified Domain Name

See FQDN.

Function

In programming, a function is part of a program that performs a specific task. Computer programs usually consist of modules of code. Each module consists of a small part of the program written to perform one specific task. These small, special-purpose chunks of code are called functions. When a program runs, it calls different functions to perform certain tasks. For example, a programmer could write a function to alphabetize a list of names. When the program got to the place where it needed to alphabetize a list of names, the program would call the alphabetizing function, and the function would return the list of names in the correct order. If those names then had to be inserted into a database, the program might call a different function to accomplish that.
See also parameter and Dynamic Link Libraries.