Failure Domain
Failure Domain is a Token Ring network area in which a failure has occurred in a Token Ring, defined by the information contained in a beacon. When a station detects a serious problem with the network (such as a cable break), it sends a beacon frame that includes the station reporting the failure, its NAUN, and everything in between. Beaconing in turn initiates a process called auto-reconfiguration.

Fallback
Fallback is a mechanism used by ATM networks when rigorous path selection does not generate an acceptable path. The fallback mechanism attempts to determine a path by selectively relaxing certain attributes, such as delay, in order to find a path that meets some minimal set of desired attributes.

Fan-in unit
The fan-in unit is a type of device that allows multiple devices on a network to communicate using a single network attachment. The opposite is called the fan-out unit, which allows a single unit to communicate with multiple units using a single network attachment. The fan-in and fan-out functions are often combined together to form the fan-in/fan-out unit, which allows a matrix communication of many-to-many units in a network using a single network attachment.

Fan-out Unit
The fan-out unit is a type of device that allows a single unit to communicate with multiple units using a single network attachment. The opposite is called the fan-in unit, which allows multiple devices on a network to communicate using a single network attachment. The fan-in and fan-out functions are often combined together to form the fan-in/fan-out unit, which allows a matrix communication of many-to-many units in a network using a single network attachment.

Fantail
Fantail, in networking, refers to the panel of I/O connectors that attaches to an equipment rack, providing easy access for data connections to a network.

FAQ: Frequently Asked Questions
Frequently Asked Questions (FAQ) is a document that addresses the frequently asked questions of users of a product or service. New users of a product or service are expected to read the FAQ before purchasing and using a product/service or participating in newsgroups, bulletin boards, video conferences, and so on. Any users are also expected to read the FAQ when they have a question regarding the product/service before they contact the customer service.

FARNET: Federation of American Research NETworks
Federation of American Research NETworks (FARNET) was a non-profit corporation, established in 1987, whose mission is to advance the use of computer networks to improve research and education. FARNET then merged with Educom's Networking and Telecommunications Task Force (NTTF), under EDUCAUSE.

Fast Ethernet
Fast Ethernet offers a speed increase ten times that of the 10BaseT Ethernet specification, while preserving such qualities as frame format, MAC mechanisms, and MTU. Such similarities allow the use of existing 10BaseT applications and network management tools on Fast Ethernet networks. Today, many network adapters support both traditional and Fast Ethernet. These so-called "10/100" adapters can usually sense the speed of the line automatically and adjust accordingly.

Fast Link Pulse
Fast Link Pulse is a type of link pulse that encodes information used in the Auto-Negotiation protocol.

Fast Packet
Fast Packet, a term in data transmission, refers to a packet that is transmitted without any error checking at points along the route. Assurance that the packet arrived without error is the responsibility of the receiver. Fast packet transmission is possible because of the extremely low incidence of error or data loss on fiber optic media and is a characteristic of high bandwidth transmission technologies such as ATM.

Fast Packet Switching
Fast packet switching is a technique that increases the switching throughput by eliminating overhead. Overhead reduction is accomplished by allocating flow control and error correction functions to either the user applications or the network nodes that interface with the user. Fast packet switching uses short, fixed length packets and, via hardware switching. ATM cell relay and frame relay are two implementations of fast packet switching.

Fast switching
Fast switching is a technique where a route cache is used to expedite packet switching through a router.

FastTrack
FastTrack is a P2P file sharing network. Known for its reliability, FastTrack handles millions of registered users with good performance of file uploads and downloads. FastTrack decentralizes the P2P file sharing process. Instead of relying on central indexing servers, FastTrack dynamically assigns indexing functions to connected peers, called "supernodes," as needed. This supernode mechanism enables scalable network performance.

Fat Pipe
A Fat Pipe in networking refers to a data transmission circuit or network that is capable of carrying large amounts of data without significantly degrading the speed of transmission. The term is derived from the simple plumbing fact that a larger-diameter pipe will carry more fluid (or gas) at a greater rate than a smaller one.

Fault Management
Fault Management is one of five categories of network management defined by ISO for network management. Fault management attempts to ensure that network faults are detected and controlled. The other nework management elements are: accounting, configuration, performance and security management.

FCAPS: Faults, Configuration, Accounting, Performance, Security
Faults, Configuration, Accounting, Performance, Security (FCAPS) is the ISO model for network management. It forms the cornerstone of current network management. FCAPS includes the five categories into which the model breaks the various network management tasks.

FCC: Federal Communications Commission
Federal Communications Commission (FCC) is an U.S. government agency that supervises, licenses, and controls electronic and electromagnetic transmission standards.

FCFS: First come first served
First come first served (FCFS) is one of the queuing algorithms used in network packet switching and routing.

FCS: Frame Check Sequence
Frame Check Sequence (FCS) refers to the extra checksum characters added to a Frame in a communication protocol for error detection and correction. The sending host computes a checksum on the entire frame and sends this along. The receiving host computes the checksum on the frame using the same algorithm, and compares it with the received FCS. In this way, it can detect whether any data was lost or altered in transit. It may then discard the data, and request retransmission of the faulty frame. A cyclic redundancy check is often used to compute the FCS.

FDDITalk
FDDITalk is the Apple Computer's data-link product that allows an AppleTalk network to be connected by FDDI cable. FDDITalk extends the data link layer to allow the AppleTalk protocol suite to operate atop a standard ANSI FDDI implementation. FDDITalk networks are organized exactly as FDDI networks, supporting the same speeds and the same number of active network nodes.

Fdgw
Fdgw is a one floppy version of NetBSD/i386. It can run on old machines without HDD and it can be used as a small router, natbox or ADSL router. It is a minimal NetBSD system.

FDM: Frequency-division Multiplexing
Frequency Division Multiplexing (FDM) is a technique by which the carrier bandwidth is divided into sub-channels of different frequency widths -- each carrying a signal at the same time in parallel. Each channel is 30 kHz. All the signals may be amplified, conducted, translated in frequency and routed toward a destination as a single signal, resulting in economies which are the motivation for multiplexing. Receivers at the receiving end separates the multiplexed signals by means of frequency passing or rejecting filters, and demodulates the results individually, each in the manner appropriate for the modulation scheme used for that band or group.

Feature Boards
Feature Boards are modular system cards that perform specific functionality, for example, DSC cards or modem cards.

FEC: Forward Error Correction
Forward Error Correction (FEC) is a technique for controlling errors in a one-way communication system. FEC sends extra information along with the data, which can be used by the receiver to check and correct the data. Using FEC, a receiver can correct errors incurred in transmission over a communications channel without requiring retransmission of any information by the transmitter, which typically involves a convolution of the transmitted bits and the appending of extra bits by both the receiver and transmitter using a common algorithm.

FEC: Forwarding Equivalence Class
A Forwarding Equivalence Class (FEC) is a term used in Multiprotocol Label Switching (MPLS) to describe a set of packets with similar or identical characteristics which may be forwarded the same way; that is, they may be bound to the same MPLS label.

FECN: Forward Explicit Congestion Notification
Forward Explicit Congestion Notification (FECN) is a bit set by a Frame Relay network to inform DTE receiving the frame that congestion was experienced in the path from source to destination. DTE-receiving frames with the FENC bit set can request that higher-level protocols take flow-control action as appropriate.

FEIP: Fast Ethernet Interface Processor
Fast Ethernet Interface Processor (FEIP) is a type of interface processor used on routers. The FEIP supports up to two 100-Mbps 100BaseT ports.

FEP: Front-end Processor
Front-End Processor (FEP) is a device or board that stands between the communications lines and the host computer to manipulate data before passing it on to the host computer for more efficient operation of the network. In the IBM SNA, it is typically an IBM 3745 device.

FFDT: FDDI Full Duplex Technology
FDDI Full Duplex Technology (FFDT), a variatoin of Fiber Distributed Data Interface (FDDI), uses the same network infrastructure as FDDI but can potentially support data rates up to 200 Mbps.

Fiber Optics
Fiber Optics is a physical method for the transmission of information (audio, video, data) using optic fiber cable and light. Light is modulated and transmitted over high purity, hair-thin fibers of glass. The bandwidth capacity of fiber optic cable is much greater than that of conventional cable or copper wire.

Fiber Plant
Fiber Plant is an aerial or buried fiber optic cable that established connectivity between fiber optic transmission equipment locations.

Fiber-optic Cable
Fiber-optic Cable is the physical medium capable of conducting modulated light transmission. Compared with other transmission media, fiber-optic cable is not susceptible to electromagnetic interference and is capable of higher data rates.

Fibre Channel
Fibre Channel, also known as Fiber Channel, is a set of related physical layer networking standards for high-performance disk storage networking of corporate networks. Fibre Channel supports functions such as data backups, clustering and replication. Fibre Channel can utilize copper wiring. However, copper limits Fibre Channel to a maximum recommended reach of 30 meters. When using more expensive fiber optic cables, Fibre Channel reaches to 10 kilometers.

FICON: Fiber Connectivity
Fiber Connectivity (FICON) is a high-speed input/output (I/O) interface for IBM mainframe computer connections to storage devices. FICON channels increase I/O capacity through the combination of a new architecture and faster physical link rates to make them up to eight times as efficient as ESCON (Enterprise System Connection) channel. FICON channels provide 100-Mbps bi-directional link rates at unrepeated distances of up to 20 km over fiber optic cables.

FID0: Format Indicator 0
Format Indicator 0 (FID0) is one of several formats that an SNA TH can use. An FID0 TH is used for communications between an SNA node and a non-SNA node.

FID1: Format Indicator 1
Format Indicator 1(FID1) is one of several formats that an SNA TH can use. An FID1 TH encapsulates messages between two subarea nodes that do not support virtual and explicit routes.

FID2: Format Indicator 2
Format Indicator 2 (FID2) is one of several formats that an SNA TH can use. An FID2 TH is used for transferring messages between a subarea node and a PU 2, using local addresses.

FID3: Format Indicator 3
Format Indicator 3 (FID3) is one of several formats that an SNA TH can use. An FID3 TH is used for transferring messages between a subarea node and a PU 1, using local addresses.

FID4 : Format Indicator 4
Format Indicator 4 (FID4) is one of several formats that an SNA TH can use. An FID4 TH encapsulates messages between two subarea nodes that are capable of supporting virtual and explicit routes.

FidoNet
FidoNet is an inter-connecting file and message transport system that was used by bulletin board systems. The network still exists today, but is dwindling in size and organization, due to the lack of callers to BBS systems that it was developed to serve, and the closing of most of those systems as a result.

Field Replaceable Unit
Field Replaceable Unit is a hardware component that can be removed and replaced on-site. Typical field-replaceable units include cards, power supplies, and chassis components.

FIFO Queueing
First-in, first-out queuing (FIFO queuing) is an algorithm in switching and routing that involves buffering and forwarding of packets in the order of arrival. FIFO embodies no concept of priority or classes of traffic. There is only one queue, and all packets are treated equally. Packets are sent out an interface in the order in which they arrive.

FIFO: First-in, first-out
First-in, first-out (FIFO) in computer networking refers to a buffering scheme where the first byte of data entering the buffer is the first byte retrieved by the CPU. In telephony, FIFO refers to a queueing scheme where the first calls received are the first calls processed.

File sharing
File sharing, also known as filesharing or file swapping, in computer networking refers to copying files from one computer to another using a live network connection. Peer to peer (P2P) filesharing is the most popular form of swapping files. P2P filesharing involves downloads of music, video and other files over the Internet. Numerous free P2P software "client" programs exist to enable this kind of filesharing.

File Transfer
File transfer is a category of network applications that allow files to be moved from one network device to another.

Filter
Filter in computer networking refers to a defined conditional test placed on incoming packets in a network system. Generally, if the packet meets the conditions defined in the filter criteria, it undergoes further processing. If the packet does not meet the filter criteria, it is rejected.

Filtering Router
Filtering Router is a type of router that selectively prevents the passage of data packets according to a specific policy.

FireFox
Firefox, also known as Mozilla Firefox, is a free, open source, cross-platform, graphical web browser developed by the Mozilla Corporation and hundreds of volunteers. Firefox includes an integrated pop-up blocker, tabbed browsing, live bookmarks, support for open standards, and an extension mechanism for adding functionality. Although other browsers have some of these features, Firefox became the first such browser to include them all and achieve wide adoption.

FireWire
FireWire is a high-performance networking standard (IEEE 1394) based on a serial bus architecture. FireWire theoretically supports networking of any computer peripherals and does not require a central computer be present on the network. FireWire competes with and is technically similar to the Universal Serial Bus (USB) standard. FireWire supports a theoretical maximum data rate of 400 Mbps, that is roughly equivalent to the performance of USB 2.0.

Firmware
Firmware is software that is embedded in a hardware device. It is often provided on flash ROMs or as a binary image file that can be uploaded onto existing hardware by a user. The firmware typically implements a portion of the network protocols, security mechanisms and administrative capabilities of the hardware device.

FISU: Fill-In Signal Unit
Fill-In Signal Unit (FISU) is an SS7 message that is sent in both directions whenever other signal units are not present. FISU provides a CRC checksum for use by both signalling endpoints.

FIX: Federal Internet Exchange
Federal Internet Exchange (FIX) is a connection point between the North American governmental internets and the Internet. The FIXs are named after their geographic region, as in FIX West (Mountain View, California) and FIX East (College Park, Maryland).

Flapping
Flapping is a type of routing problems where an advertised route between two nodes alternates (flaps) back and forth between two paths due to a network problem that causes intermittent interface failures.

Flash Update
Flash Update is a type of routing update sent asynchronously in response to a change in the network topology.

Flat Addressing
Flat Addressing is a scheme of addressing that does not use a logical hierarchy to determine location. For example, MAC addresses are flat, so bridging protocols must flood packets throughout the network to deliver the packet to the appropriate location.

F-link: Fully associated link
F-link refers to the SS7 fully associated link, which is an SS7 signaling link directly associated with a link carrying traffic. F-link connects two signaling end points (i.e., SSPs and SCPs). F-links are not usually used in networks with STPs. In networks without STPs, F-links directly connect signaling points.

Flooding
Flooding in networking refers to a situation in a network device when incoming packets exceeded its processing capacity. When flooding happens, the device may get into the failopen mode or simply drop packets.

Flow
Flow in networking refers to the stream of data traveling between two endpoints across a network. Multiple flows can be transmitted on a single circuit.

Flow Control
Flow Control is the process of controlling data transmission at the sender to avoid overfilling buffers and loss of data at the receiver. This may be either software flow control or hardware flow control. The receiver typically has a fixed size buffer into which received data is written as soon as it is received. When the amount of buffered data exceeds a "high water mark", the receiver will signal to the transmitter to stop transmitting until the process reading the data has read sufficient data from the buffer that it has reached its "low water mark", at which point the receiver signals to the transmitter to resume transmission. There are many flow control mechanisms. One of the most common flow control protocols for asynchronous communication is called xon-xoff.

Flowspec
In MPLS and RSVP-TE protocols, Flowspec is the traffic parameters of a stream of IP packets between two applications.

FLT: Full Line Terminal
Full Line Terminal (FLT) is a multiplexer that terminates a SONET span.

FNC: Federal Networking Council
Federal Networking Council (FNC) is a US government group responsible for assessing and coordinating U.S. federal agency networking policies and needs.

FOIRL: Fiber-optic Interrepeater Link
Fiber-optic Interrepeater Link (FOIRL) is a fiber-optic signaling methodology based on the IEEE 802.3 fiber-optic specification. FOIRL is a precursor of the 10BaseFL specification, which is designed to replace it.

Footprint
Footprint in networking and telecom means the physical size of a networking, telecom or computing equipment.

Ford-Fulkerson Algorithm
Ford-Fulkerson Algorithm, also known as Bellman-Ford Routing Algorithm, is used as an algorithm by distance vector routing protocols such as RIP, BGP, ISO IDRP, NOVELL IPX. Routers that use this algorithm have to maintain the distance tables (which is a one-dimension array -- "a vector"), which tell the distances and shortest path to sending packets to each node in the network. The information in the distance table is always updated by exchanging information with the neighboring nodes. The number of data in the table equals to that of all nodes in networks (excluded itself). The columns of table represent the directly attached neighbors whereas the rows represent all destinations in the network. Each data contains the path for sending packets to each destination in the network and distance/or time to transmit on that path (we call this as "cost"). The measurements in this algorithm are the number of hops, latency, the number of outgoing packets, etc.

Forward Channel
Forward Channel is a communications path or a data circuit carrying information from the original user to the destination user.

Forward Delay Interval
Forward Delay Interval is the amount of time an interface spends listening for topology change information after that interface is activated for bridging and before forwarding actually begins.

Forward Delay Timer
Forward Delay Timer is one of the Spanning Tree Protocol (STP) timer that sets the amount of time an interface spends in the STP listening and learning states. In other words, the Forward Delay timer specifies the time a port stays in each of the listening and learning states.

Forward Lookup
Forward lookup means using an Internet domain name to find an IP address.

Forward or forwarding
Forward in networking refers to sending or moving a frame toward its ultimate destination through internetworking devices.

Forwarding State
Forwarding State is a Spanning Tree Protocol (STP) port state in which the bridge or switch processes frames (input or output) on the interface, with the exception of STP messages.

FOTS: Fiber Optics Transmission Systems
Fiber Optics Transmission Systems (FOTS) is a type of data transmission using electromagnetic energy in the form of light waves. Such systems are termed to be photonic, although they actually are Electro-Opto-Electronic (EOE) in nature, as the signal both originates and terminates in systems that are electrically based. FOTS generally are digital in nature, although in rare instances they can be analog.

Fourier Transform
The Fourier transform, named after Joseph Fourier, is an integral transform that re-expresses a function in terms of sinusoidal basis functions, i.e. as a sum or integral of sinusoidal functions multiplied by some coefficients ("amplitudes"). Fourier Transform technique is used to evaluate the importance of various frequency cycles in a time series pattern.

FQDN: Fully Qualified Domain Name
Fully Qualified Domain Name (FQDN) is the full name of a system, rather than just its host name. For example, NetworkDictionary is a host name, and www.NetworkDictionary.com is an FQDN.

FRAD: Frame Relay Access Device
Frame Relay Assembler/Disassembler (FRAD), also known as Frame Relay Access Device, is a communications device in a Frame Relay router which is used to interface a LAN with a frame relay WAN. FRAD breaks a data stream into frames for transmission over a Frame Relay network and recreates a data stream from incoming frames.

Fragment
Fragment is a piece of a larger packet that has been broken down to smaller units. For example, when a router is forwarding an IP packet to a network that has a maximum packet size smaller than the packet size, it is forced to break up that packet into multiple fragments. These fragments will be reassembled by the IP layer at the destination host.

Fragment Offset
The fragment offset is a field in a fragment header that tells the receiver where a particular fragment falls in relation to other fragments in the original larger packet.

Fragmentation
Fragmentation is the process of breaking a packet into smaller units (fragments) when transmitting over a network medium that cannot support the original size of the packet. Fragmentation also refers to the process of storing a data file in several "chunks" or fragments rather than in a single contiguous sequence of bits in one place on the storage medium. The reverse process is called "defragmentation".

Fragment-free Switching
Fragment-free switching, also known as runtless switching, is a hybrid of cut-through and store-and-forward switching. Fragment-free switching was developed to solve the late-collision problem. Fragment-free switching is suitable for backbone applications in a congested network, or when connections are allocated to a number of users. The switching device checks the source and destination MAC address of a packet, and sends the packet to the port corresponding to the destination.

Frame
Frame refers to the unit of information sent at the data-link layer over a transmission medium. A frame contains the header, the trailer, as well as the user data in the unit. The terms of cell, datagram, message, packet, and segment also are used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

Frame Forwarding
Frame Forwarding is a mechanism by which frame-based traffic, such as HDLC and SDLC, traverses an ATM network.

Frame Relay Bridging
Frame Relay Bridging is a bridging technique that uses the same spanning-tree algorithm as other bridging functions but allows packets to be encapsulated for transmission across a Frame Relay network.

Framing
Framing is a process at the physical and data-link layers of the OSI model, in which bits are fit into units called frames. Frames contain source and destination information, flags to designate the start and end of the frame, plus information about the integrity of the frame. All information from the above layer is viewed as a payload of data and is encapsulated in the frame.

Freenet
Freenet is a decentralized censorship-resistant peer-to-peer distributed data store. Freenet works by pooling the contributed bandwidth and storage space of member computers to allow users to anonymously publish or retrieve various kinds of information. Freenet uses a kind of key based routing similar to a distributed hash table to locate peers' data.Freenet is currently under development, and a version 1.0 has not yet been released. Freenet is considered by many to be fundamentally different from other peer-to-peer networks; it is still somewhat more difficult to use, slower, and does not have integrated search functionality.

FREESCO: FREE ciSCO
FREE ciSCO(FREESCO) is a free replacement for the low end commercial routers. FREESCO is open source, stable, inexpensive, easy to use, extremely versatile and flexible ... and best of all, FREESCO is FREE. FREESCO is based on the Linux operating system. And incorporates many of the features of other Linux distributions into software that fits onto a single 1.44 meg floppy diskette.

Free-trade Zone
Free-trade Zone is part of an AppleTalk internetwork that is accessible by two other parts of the internetwork that cannot directly access one another.

FRF.11
FRF.11 is the Frame Relay Forum implementation agreement for Voice over Frame Relay. This specification defines multiplexed data, voice, fax, DTMF digit-relay, and CAS/Robbed-bit signaling frame formats but does not include call setup, routing, or administration facilities.

FRF.12 Implementation Agreement
The FRF.12 Implementation Agreement, also known as FRF.11 Annex C, was developed to allow long data frames to be fragmented into smaller pieces and interleaved with real-time frames. In this way, real-time voice and non real-time data frames can be carried together on lower speed links without causing excessive delay to the real-time traffic.

FRF.8
FRF.8 is the specification for the Frame Relay-to-ATM Service Interworking. To communicate over WANs, end-user stations, the network cloud typically must use the same type of transmission protocol. This limitation has prevented differing networks, such as Frame Relay and ATM, from being linked. However, the Frame Relay-to-ATM Service Interworking (FRF.8) feature allows Frame Relay and ATM networks to exchange data despite differing network protocols. The Frame Relay/ATM PVC Service Interworking Implementation Agreement specified in Frame Relay Forum (FRF) document number FRF.8 provides the functional requirements for linking Frame Relay and ATM networks.

FRF: Frame Relay Forum
Frame Relay Forum (FRF) was an association of vendors, carriers, users, and consultants committed to the implementation of Frame Relay in accordance with national and international standards. Now the FRF has become part of the MFA (MPLS, Frame Relay and ATM) Forum.

FRF11-trunk
FRF11-trunk refers to the point-to-point permanent voice connection (private line) conforming to the FRF.11 specification.

Friend-to-friend (F2F)
A friend-to-friend (F2F) refers to a special type of Peer-to-peer (P2P) computer network in which people use direct connections with their "friends". F2F software only allows people you trust (using IP addresses or digital signatures you trust) to exchange files directly with your computer. Then your friends' own friends (and so on) can indirectly exchange files with your computer, never using your IP address.

Front End
Front End refers to a node or software program, such as a web browser, that requests services of a back end such as a database.

FRTS: Frame Relay Traffic Shaping
Frame Relay Traffic Shaping (FRTS) is a queueing method that uses queues on a Frame Relay network to limit surges that can cause congestion. Data is buffered and sent into the network in regulated amounts to ensure that the traffic can fit within the promised traffic envelope for the particular connection.

FTTH: Fiber-To-The-Home
Fiber to the Home (FTTH), also known as Fiber to the Premises (FTTP), refers to a broadband telecommunications system based on fiber-optic cables and associated optical electronics for delivery of multiple advanced services such as the triple play of telephone, broadband Internet and television to homes and businesses.

FTTP: Fiber-To-The-Premises
Fiber-To-The-Premises (FTTP), also known as Fiber-To-The-Home (FTTH), refers to a broadband telecommunications system based on fiber-optic cables and associated optical electronics for delivery of multiple advanced services such as the triple play of telephone, broadband Internet and television to homes and businesses.

Full Duplex
Full Duplex transmission indicates the transmission of data in two directions simultaneously. For example, on a local area network with full-duplex transmission, one workstation can be sending data on the line while another workstation is receiving data. On the other hand, a half duplex transmission indicates that data transmission happens only in one direction at a time.

Full Mesh
Full mesh is a type of network topology which occurs when every node has a circuit connecting it to every other node in a network. Full mesh is very expensive to implement but yields the greatest amount of redundancy, so in the event that one of those nodes fails, network traffic can be directed to any of the other nodes. Full mesh is usually reserved for backbone networks.

Full-Duplex Operation
Full-Duplex Operation, also known as Full Duplex transmission, is a communication method that allows for the simultaneously transmission and reception of data.

Fully-Qualified Domain Name
A Fully-Qualified Domain Name is a server name with a hostname followed by the full domain name.

Function Group
Function Group is an ISDN term that generically refers to a set of functions that a piece of hardware or software must perform. Because the ITU wanted several options for the customer, it defined multiple different function groups.

FUNET: Finnish University Network
Finnish University Network (FUNET) is a backbone network providing Internet connections for Finnish universities.

Fuzzball Router
Fuzzball routers were the first modern routers on the Internet. They were DEC LSI-11 computers loaded with router software written by David L. Mills of the University of Delaware. About fifty of them were deployed worldwide in the early 1980s to test many of the Internet's first protocols. The NSFnet used these systems as backbone packet switches.

FXS: Foreign Exchange Station
Foreign Exchange Station (FXS) is a telephone interface which provides battery power, sends dialtone, and generates ringing voltage. A standard telephone plugs into such an interface to receive telephone service. A telephone exchange is an example of an FXS.