H.323

A standard approved by the International Telecommunications Union (ITU) that defines how audiovisual conferencing data is transmitted across networks.

HAL

See Hardware Abstraction Layer.

Half-Duplex

A method of transmitting information over a communication channel, in which signals may be sent in both directions, but only one way at a time. This is sometimes referred to as local echo.

Hand-Held Computer

A portable computer that is small enough to be held in one hand, such as the enormously popular PalmPilot from 3Com Corporation.

Handshaking

In network communication, a process used to verify that a connection has been established correctly. Devices send signals back and forth to establish parameters for communication.

Hard Costs

Costs that can be easily identified and quantified, such as the cost of hardware or software.

Hard Disk

A device built into the computer for holding programs and data. It is sometimes referred to as the fixed disk.

Hard Disk Drive

Hard disk drives are mass storage devices that read and write digital information that is stored on spinning disks. The spinning disks must be precisely aligned and cannot normally be removed. Hard disk drives are an inexpensive way to store gigabytes of computer data permanently. See also Mass Storage Device.

Hard Disk Partitioning

The process of dividing one physical disk drive into several logical disk drivers, each called partitions and each having a separate drive letter.

Hardcopy

Output on paper.

Hardware

The equipment that makes up a computer system, excluding the programs or software.

Hardware Abstraction Layer (HAL)

A Windows Server 2003 service that provides basic input/output services such as timers, interrupts, and multiprocessor management for computer hardware. The HAL is a device driver for the CPU/motherboard circuitry that allows different families of computers to be treated the same by the Windows Server 2003 operating system. See also Driver, Service, and Interrupt Request.

Hardware Address

See Media Access Control (MAC) Address.

Hardware Compatibility List (HCL)

The listing of all hardware devices supported by Windows NT. Hardware on the HCL has been tested and verified as being compatible with NT. You can view the current HCL at http://microsoft.com/ntserver/hcl.

Hardware Profile

A file that stores a hardware configuration for a computer. Hardware profiles are useful when a single computer (a laptop that can be docked or undocked) has multiple hardware configurations.

Hashing Algorithm

A special type of algorithm, also known as a message authentication algorithm, used to create a digital signature. It calculates a unique mathematical value, known as a digest, from the actual message contents. See also Digest.

H Channel

Also known as High-Speed Channel. A full-duplex, ISDN primary rate channel operating at a speed of 384Kbps. See also B Channel, D Channel, and E Channel.

HCL

See Hardware Compatibility List.

HDLC

See High-Level Data Link Control.

HELP

Provides complete, interactive Help information for MS-DOS commands.

Help

A feature that gives you instructions and additional information on using DOS (DOS applications normally include their own help systems).

Helper Address

1. The address that is configured on an interface to which broadcasts that are received by that interface will be sent.
2. The unicast address specified, which instructs the Cisco router to change the client’s local broadcast request for a service into a directed unicast to the server.

Heuristic Analysis

Behavior-based analysis of a computer program by anti-virus software to identify a potential virus. Often heuristic scanning produces false alarms when a clean program behaves as a virus might. Also known as Heuristic Scan.

Hibernation

The storage of anything that is stored in memory on the computer’s hard disk. Hibernation ensures that none of the information stored in memory is lost when the computer is shut down. When the computer is taken out of hibernation, it is returned to its previous state.

Hidden Files

Files that do not normally appear in a directory listing, such as the IO.SYS and MSDOS.SYS files.

Hierarchical Addressing

Any addressing plan employing a logical chain of commands to determine location. IP addresses are made up of a hierarchy of network numbers, subnet numbers, and host numbers to direct packets to the appropriate destination.

Hierarchical Naming

A method of naming objects in which a layered structure, often referred to as a tree, is built to organize resources.

Hierarchy

Any structure or organization that uses class, grade, or rank to arrange objects.

High-Level Data Link Control (HDLC)

Using frame characters, including checksums, HDLC designates a method for data encapsulation on synchronous serial lines and is the default encapsulation for Cisco routers. HDLC is a bit-oriented synchronous Data-Link layer protocol created by ISO and derived from SDLC. However, most HDLC vendor implementations (including Cisco’s) are proprietary. See also Synchronous Data Link Control.

High-Level Security

A type of DNS security that uses all of the DNS security features of medium-level security, and adds the advanced security that is available when you run DNS servers on domain controllers and use Active Directory-Integrated zones. In this configuration, there is no DNS communication at all with the Internet.

Highlight

The change to a reverse-video appearance when a menu item or area of text is selected.

High Performance File System (HPFS)

The file system native to OS/2 that performs many of the same functions of NTFS when run under OS/2. See also File System and New Technology File System.

High-Speed Communication Interface (HSCI)

Developed by Cisco, a single-port interface that provides full-duplex synchronous serial communications capability at speeds up to 52Mbps.

High-Speed Serial Interface (HSSI)

A network standard physical connector for high-speed serial linking over a WAN at speeds of up to 52Mbps.

Hijacking

An attack whereby an active, established, session is intercepted and used by the attacker. Hijacking can occur locally if, for example, a legitimate user leaves a computer unprotected. Remote hijacking can occur via the Internet.

HIMEM.SYS

The system file that allows the use of extended memory by DOS programs.

HIP

See HSSI Interface Processor.

HMA

High Memory Area; the first 64KB of memory beyond the end of the base memory, which DOS software can access.

Holddown

The state a route is placed in so that routers can neither advertise the route nor accept advertisements about it for a defined time period. Holddown is used to surface bad information about a route from all routers in the network. A route is generally placed in holddown when one of its links fails.

Hole

Vulnerability in the design software and/or hardware that allows circumvention of security measures.

Home Directory

A directory that stores a user’s personal files.

Home Folder

A folder where users normally store their personal files and information. A home folder can be a local folder or a network folder.

Home Page

The default page returned by an HTTP server when a URL containing no specific document is requested. See also Hypertext Transfer Protocol and Uniform Resource Locator.

Home Server

The Exchange server on which an object physically resides.

Hop

In routing, a server or router that is counted in a hop count.

Hop Count

This method describes the number of routers that a message might cross before it reaches its destination. If all hops are assumed to take the same amount of time, the optimum path is the path with the smallest hop count.

Horizontal Partitioning

In SQL Server replication, a method by which you can publish only certain rows of a table. This is often referred to as horizontal filtering. See also Vertical Partitioning.

Host

An addressable computer system on a TCP/IP network. Examples would include endpoint systems such as workstations, servers, minicomputers, mainframes, and intermediate systems such as routers. A host is typically a system that offers resources to network nodes. In remote access the host is the computer providing data to the RAS client and hosting its connection to the remote network. See also Remote Access Service.

Host Address

Logical address configured by an administrator or server on a device. Logically identifies this device on an internetwork.

Host Name

The TCP/IP command that returns the local workstation’s host name used for authentication by TCP/IP utilities. This value is the workstation’s computer name by default, but it can be changed by using the Network icon in Control Panel.

Host Record

Associates a host’s name to its IP addresses. Also known as an address or A record.

Host Route

Provides a route to a single system; normally used when you want to direct traffic to remote networks through a particular machine.

Host Server

In the context of the Distributed file system (Dfs), the domain server that contains the Dfs root. The host server automatically publishes the Dfs topology to the Active Directory and provides synchronization of the topology across the domain member servers. See also Distributed File System and Dfs Root.

HOSTS File

A file that is used to map IP addresses to host names. A HOSTS file can be used in place of a DNS server. See also IP Address, Host, and DNS Server.

Host Table

The HOSTS or LMHOSTS file that contains lists of known IP addresses.

Hosted Organization

Also known as Virtual Server, Virtual Machine, and Virtual Organization. A collection of Exchange services including, but not limited to virtual servers (that is, instances of Internet Message Access Protocol 4 (IMAP4), SMTP, POP3, NNTP, HTTP, RVP), storage space, and real-time collaboration facilities that exist to serve the needs of a single company. A hosted organization is normally used by Internet Service Providers to host multiple companies on the same physical computer. However, a hosted organization is not limited to a single Exchange 2000 server.

Host-to-Host Layer

The DoD model layer that references to the Transport layer of the OSI model.

Hot Standby Router Protocol (HSRP)

A protocol that provides high network availability and provides nearly instantaneous hardware fail-over without administrator intervention. It generates a Hot Standby router group, including a lead router that lends its services to any packet being transferred to the Hot Standby address. If the lead router fails, it will be replaced by any of the other routers – the standby routers – that monitor it.

Hot Swapping

The abbility of a device to be plugged into or removed from a computer while the computer’s power is on.

HPFS

See High Performance File System.

HSCI

See High-Speed Communication Interface.

HSRP

See Hot Standby Router Protocol.

HSSI

See High-Speed Serial Interface.

HSSI Interface Processor (HIP)

An interface processor used on Cisco 7000 series routers, providing one HSSI port that supports connections to ATM, SMDS, Frame Relay, or private lines at speeds up to T3 or E3.

HTML

See Hypertext Markup Language.

HTTP

See Hypertext Transfer Protocol.

HTTP-DAV

See Distributed Authoring and Versioning (DAV).

HTTP Redirector Filter

Intercepts a web request for a specified destination from a client and then redirects it to an alternate site as determined by the administrator.

Hub

An Ethernet Data Link layer device that connects point-to-point Physical layer links, such as twisted pair or fiber-optic cables, into a single shared media network. See Data Link Layer, Ethernet, Active Hub, Intelligent Hub, and Passive Hub.

Hurd

A project from the Free Software Foundation (FSF) to develop and distribute a free version of the Unix operating system for many different hardware platforms. Hurd (or sometimes HURD) is considered a collection of all the GNU software, compilers, editors, and utilities, as well as the operating system. See also Free Software Foundation and GNU.

Hyperlink

A link in text or graphics files that have a Web address embedded within them. By clicking the link, you jump to another Web address. You can identify a hyperlink because it is a different color than the rest of the Web page. See also World Wide Web.

Hypertext Markup Language (HTML)

The script language used to create WWW content. HTML can create hyperlinks between objects on the WWW. See also Hypertext Transfer Protocol and World Wide Web.

Hypertext Transfer Protocol (HTTP)

Hypertext transfer protocol is an Internet protocol that transfers HTML documents over the Internet and responds to context changes that happen when a user clicks on a hypertext link. See also Hypertext Markup Language and World Wide Web.