Hairpin
Hairpin means to send a call back in the direction that it came from. For example, in a VOIP network, if a call cannot be routed over IP to a gateway that is closer to the target telephone, the call typically is sent back out the local zone, back the way from which it came.

Half-Duplex
Half-duplex is a data transmission mothod in which data can be transmitted in both directions on a signal carrier, but not at the same time. For example, on a local area network using a technology that has half-duplex transmission, one workstation can send data on the line and then immediately receive data on the line from the same direction in which data was just transmitted. On the other hand, full-duplex transmission implies that data are transmitted in both directions simultaneously.

Half-Duplex Operation
Half-duplex operation means doing data transmission using half duplex method in which data can be transmitted in both directions on a signal carrier, but not at the same time. On the other hand, full-duplex transmission implies that data are transmitted in both directions simultaneously.

Handshake or Handshaking
In the context of networking and telecommunication, handshake or handshaking is a process to establish communications in which sequence of messages are exchanged between two or more network devices to ensure transmission synchronization. Hardware handshaking signals between two computers are carried by voltage levels or pulses on wires. There are many handshaking protocols for software handshaking. Basically, the handshaking protocol send a "hello" message to the receiving system and get a "ok" responce. When the communication between the two systems finished, a "finish" message should be send and confirmed by both systems.

Handshake
Handshake is a process to establish communications in which sequence of messages are exchanged between two or more network devices to ensure transmission synchronization.

Handshaking
In the context of computer networking, handshaking indicates hardware or software to establish or maintain two machines or programs in synchronisation. Handshaking often requires the exchange of messages or packets of data between two systems. Hardware handshaking signals between two computers which are carried by voltage levels or pulses on wires. There are many handshaking protocols for software handshaking. Basically, the handshaking protocol send a "hello" message to the receiving system and get a "ok" responce. When the communication between the two systems finished, a "finish" message should be send and confirmed by both systems.

Headend
Headend, alo known as head-end, is the end point of a network where all stations transmit toward the headend; the headend then transmits toward the destination stations. In a cable network, headend is the central distribution point for a CATV system. Video signals are received here from satellites and maybe other sources, frequency converted to the appropriate channels, combined with locally originated signals, and rebroadcast onto the HFC plant.

Header
Header refers to the top part of a message which maybe a packet, frame, cell or a document, URL Header is very critical for any communication protocols in which the control information is placed before data when encapsulating that data for network transmission.

HEAnet: Ireland's National Education & Research Network
Ireland's National Education & Research Network(HEAnet) provides high-speed Internet access to academic institutions in Ireland direct to European and USA networks. The network is funded by the Higher Education Authority and is a WAN-based network.

HEC: Header Error Control
Header Error Control (HEC) is algorithm for checking and correcting an error in an ATM cell using the fifth octet in the ATM cell header. The check character is calculated using a Cyclic Redundancy Check (CRC) algorithm allowing a single bit error in the header to be corrected or multiple errors to be detected.

Hello
Hello (should not be confused with the HELLO protocol) is a process used by some protocols as the handshaking method to eatablish and maintain relationships. For example, Open Shortest Path First (OSPF) systems uses Hello to establish and maintain neighbor relationships. Hello is also used in the STP Hello BPDU message in a Spanning Tree.

Hello Message
Hello messages, also known as hello packets, are the multicast packets that are used by routers for neighbor discovery and recovery. Hello packets also indicate that a client is still operating and network-ready.

Hello Packet
Hello Packets, also known as hello messages, are the multicast packets that are used by routers for neighbor discovery and recovery. Hello packets also indicate that a client is still operating and network-ready.

Hello Timer
Hello Timer is a Spanning Tree Protocol (STP) timer that dictates how often the root bridge or switch sends STP hello messages. It also dictates how often nonroot bridges and switches should expect to hear these hello messages. This term may also refer to the Hello timer used by OSPF, which defines how often a router sends OSPF Hello messages.

Helper address
Helper address is the address configured on an interface to which broadcasts received on that interface will be sent.

HEPnet
High-Energy Physics Network (HEPnet) is the research network that originated in the United States, but that has spread to most places involved in high-energy physics. Well-known sites include Argonne National Laboratory, Brookhaven National Laboratory, and the Stanford Linear Accelerator Center (SLAC).

Heterogeneous Network
Heterogeneous Network is a type of network consisting of dissimilar devices that run dissimilar protocols and in many cases support dissimilar functions or applications.

Heuristic Routing
Heuristic Routing is a type of routing algorithm in which data, such as time delay, extracted from incoming messages, during specified periods and over different routes, are used to determine the optimum routing for transmitting data back to the sources.

HFC: Hybrid Fiber Coax
Hybrid Fiber Coax (HFC) is a technology of delivering video, voice telephony, data, and other interactive services over coaxial and fiber optic cables. An HFC network consists of a head-end office, distribution center, fiber nodes, and network interface units. The head-end office receives information such as television signals, Internet packets, and streaming media, and then delivers them through a SONET ring to distribution centers. The distribution centers then send the signals to neighborhood fiber nodes, which convert the optical signals to electrical signals and redistribute them on coaxial cables to residents' homes where network interface units send the appropriate signals to the appropriate devices (i.e. television, computer, telephone).

Hierarchical Addressing
Hierarchical Addressing is a scheme of addressing that uses a logical hierarchy to determine location. For example, IP addresses consist of network numbers, subnet numbers, and host numbers, which IP routing algorithms use to route the packet to the appropriate location.

Hierarchical Routing
Hierarchical Routing is a routing strategy on large networks by reducing the size of the networks into a hierarchy of networks, where each level is responsible for its own routing.

HIL Bus: Human Interface Loop Bus
Human Interface Loop Bus (HIL Bus) is a computer bus used by Hewlett-Packard (HP) to connect keyboards, mice and less common human interface peripherals to their HP 9000 workstations. The bus was in use until the mid-1990s, when HP substituted PS/2 technology for HIL. The PS/2 peripherals were themselves replaced with USB-connected models.

HIP: HSSI Interface Processor
High-Speed Serial Interface (HSSI) Interface Processor (HIP) provides one HSSI port that supports connections to ATM, SMDS, Frame Relay, or private lines at speeds up to T3 or E3.

HIPPI: HIgh Performance Parallel Interface
HIgh Performance Parallel Interface(HIPPI) is a computer bus for the attachment of high speed storage devices to supercomputers. It was popular in the late 1980s and into the mid-to-late 1990s, but has since been replaced by ever-faster standard interfaces like SCSI and Fibre Channel.

HLA: High Level Architecture
The High Level Architecture (HLA) is a general purpose architecture for distributed computer simulation systems. Using HLA, computer simulations can communicate to other computer simulations regardless of the computing platforms. Communication between simulations is managed by a runtime infrastructure (RTI).

HLD: High-level Designator
High-level Designator (HLD) indicates the entry or exit point of a block in the network. HLD logically identifies the peer session endpoints used if the multiplex in the circuit is set to group.

HLP: Higher Layer Protocol
Higher Layer Protocol (HLP), also known as uppler layer protocol (ULP), typically refers to the protocols that operate at the top layers in the OSI reference model, such as layers 4-7. HLP is sometimes used to refer to the next-highest protocol (relative to a particular protocol) in a protocol stack.

Holddown
Holddown is a state of a route when the router will neither advertise the route nor accept advertisements about the route for a specific length of time (the holddown period). Holddown is used to flush bad information about a route from all routers in the network. A route is typically placed in holddown when a link in that route fails.

Holddown period
Holddown period is the time interval when a router is in the holddown state. During the period, the router will neither advertise the route nor accept advertisements about the route.

Home Gateway
The home gateway is a router or access server ensuring continuity between the home network and the in-home connected devices and the external world represented by a wide area network (WAN) or the Internet.

Homologation
Homologation is derived from the Greek homologos for "agree". In computer networking, homologation refers to the conformity of a product or a specification to international standards, such as ITU-T, IETF, IEEE, CSA, TUV, UL, or VCCI, which enables portability and interopability across company and international boundaries.

Hoot and Holler
Hoot and Holler is a type of broadcast audio network used extensively by the brokerage industry for market updates and trading. Similar networks are used in publishing, transportation, power plants, and manufacturing.

Hop
In computer networking, a hop is each exchange with a router/switch a packet takes on its way to the destination. When communicating over the Internet, for example, data passes through a number of intermediate devices (like routers) rather than flowing directly over a single wire. Each such device causes data to "hop" between network connections step by step.

Hop Count
In networking, the hop count represents the total number of devices a given piece of data (packet) passes through. Generally speaking, the more hops data must traverse to reach their destination, the greater the transmission delay incurred. Hop count is one of routing metric used to measure the distance between a source and a destination. Some routing protocols such as Routing Information Protocol (RIP) use hop count as its sole metric.

Hop Off
In traditional telecom, Hop Off refers to a point at which a signal or call leaves a network and moves to another network. For example, a call made on a phone set can hop off the local telephone network to long distance network. Conversely, a call coming through a major network can hop off that network to a smaller local network or private branch exchange (PBX). In voice over IP (VoIP), Hop Off is a point or gateway at which a call moves from an H.323 network to a network that uses some other protocol, typically at a gateway.

Host
Host refers to a computer system on a network. Host is similar to node in some sense, except that host usually implies only computer systems, whereas node generally applies to any networked devices, including access devices and routers.

Host address
Host address, also known as host number, is a fully qualified domain name (usually alphabetic) identifying the address of one specific host computer on the Internet. The host address is a subset of the IP address.

Host File
A host file, stored on the computer's file-system, is used to look up the Internet Protocol address of a device connected to a computer network. The host file describes a many-to-one mapping of device names to IP addresses. When accessing a device by name, the networking system will attempt to locate the name within the host file if it exists. Typically, this is used as a first means of locating the address of a system, before accessing the Internet domain name system.

Host Name
Host Name, also known as hostname, is the name given to a computer system. The hostname is used to identify a particular host in various forms of electronic communication such as E-mail or Usenet. Hostnames are used by various naming systems: NIS, DNS, SMB, etc.

Host Node
Host Node is an IBM SNA subarea node that contains a system service control point(SSCP). A host node is a type 5 node, typically a mainframe computer compatible with the original IBM System/370.

Host Number
Host Number, also known as host address, is a subset of an IP address that designates which node on the subnetwork is being addressed.

Hot Standby
Hot Standby is redundancy technology in which the primary and backup systems run simultaneously. The data is mirrored to the backup system in real time so that both systems contain identical information. By doing that, the backup system is ready to take over the processing instantaneously, should there be any failure in the primary system.

Hot Swapping
Hot Swapping is the process of replacing a hub module without bringing down the network. This process occurs by sliding an active module into a fully powered up concentrator, replacing a failed module.

Hot-potato Routing
Hot-potato routing is the practice of passing traffic off to another autonomous system (AS) as quickly as possible, thus using their network for wide-area transit. On the other hand, in Cold-potato routing, the originating AS holds onto the packet until it is as near to the destination as possible.

HPCC: High Performance Computing and Communications
High Performance Computing and Communications (HPCC) is a U.S. government funded program advocating advances in computing, communications, and related fields. The HPCC is designed to ensure US. leadership in these fields through education, research and development, industry collaboration, and implementation of high-performance technology.

HPCS: High-Performance Computing Systems
High-Performance Computing Systems (HPCS) is a component of the US High Performance Computing and Communications (HPCC) program.

HPPI: High Performance Parallel Interface
High Performance Parallel Interface (HPPI) is a standard point-to-point protocol for transmitting large amounts of data at up to billions of bits per second over relatively short distances, mainly on local area networks (LANs). The proponents of HIPPI believe that its use can make computers, interconnected storage devices, and other resources on a local area network function as though they were all within a single supercomputer.

HPR: High Performance Routing
High Performance Routing(HPR), a second-generation routing algorithm for APPN, provides a connectionless layer with nondisruptive routing of sessions around link failures, and a connection-oriented layer with end-to-end flow control, error control, and sequencing.

HSCI: High-Speed Communications Interface
High-Speed Communications Interface (HSCI) is a single-port interface, developed by Cisco, providing full-duplex synchronous serial communications capability at speeds up to 52 Mbps.

HSSI: High-Speed Serial Interface
The High-Speed Serial Interface (HSSI) is a DTE/DCE interface that was developed by Cisco Systems and T3 plus Networking to address the need for high-speed (up to 52 Mbps) communications over WAN links. The HSSI specification is available to any organization wanting to implement HSSI.

HT: HyperTransport
HyperTransport (HT), formerly known as Lightning Data Transport (LDT), is a bidirectional serial/parallel high-bandwidth, low-latency computer bus. The HyperTransport Technology Consortium is in charge of promoting and developing HyperTransport technology.

Hub
The Hub (or repeater) is a device that accepts numerous Ethernet connections from network devices and cross-connects them. Data arriving via the receive pair of one connection is regenerated and sent out on the transmit pair to all connected devices except for the device that originated the transmission.

Hub MIB
Hub MIB or HUBMIB is a specification that defines a set of managed objects that instrument devices, MAUs and interfaces that conform to the IEEE 802.3 standard for Ethernet. In particular, it defines objects for managing IEEE 802.3 10 Mbpsecond baseband hubs or repeaters.

Hybrid network
Hybrid network is an internetwork made up of more than one type of network technology, including LANs and WANs.

Hyperlink
Hyperlink is a pointer within a hypertext or hypermedia or other documents that points (links) to another document, which might or might not be a hypertext document.

Hypertext
Hypertext is a type of electronically stored text that allows "click through" access to other texts by way of encoded links (hyperlinks). Hypertext documents can be created using HTML, PHP or other formats, and often integrate images, sound, and other media that are commonly viewed using a browser. Hypertext is the basic concept behind the WWW, whereby one resource can be linked to other relevant information elsewhere on the WWW.