D

DAC

See Discretionary Access Control (DAC).

data integrity

A quality that provides a level of confidence that data won't be jeopardized and will be kept secret.

Data Link layer

The second layer of the Open Systems Interconnection (OSI) model. It describes the physical topology of a network.

data packet

A unit of data sent over a network. A packet includes a header, addressing information, and the data itself.

data repository

A centralized storage location for data, such as a database.

data source

Where data originates.

datagram

A Layer 3, User Datagram Protocol (UDP) packet descriptor.

DDoS attack

See distributed denial of service (DDoS) attack.

decryption

The process of converting encrypted data back into its original form.

default gateway

The router to which all packets are sent when the workstation doesn't know where the destination station is or when it can't find the destination station on the local segment.

demilitarized zone (DMZ)

An area for placing web and other servers that serve the general public outside the firewall, therefore, isolating them from internal network access.

denial of service (DoS) attack

A type of attack that prevents any users—even legitimate ones—from using a system.

destination port number

A portion of a complete address of a PC to which data is being sent from a sending PC. The port portion allows for the demultiplexing of data to be sent to a specific application.

detection

The act of noticing an irregularity as it occurs.

DHCP

See Dynamic Host Configuration Protocol (DHCP).

dictionary attack

The act of attempting to crack passwords by testing them against a list of dictionary words. With today's powerful computers, an attacker can combine one of many available automated password-cracking utilities with several large dictionaries or "wordlists" and crack huge numbers of passwords in a matter of minutes. Any password based on any dictionary word is vulnerable to such an attack.

differential backup

A type of backup that includes only new files or files that have changed since the last full backup. Differential backups differ from incremental backups in that they don't clear the archive bit upon their completion.

Diffie-Hellman

An asymmetric standard for exchanging keys. This cryptographic algorithm is used primarily to send secret keys across public networks. The process isn't used to encrypt or decrypt messages; it's used merely for the transmission of keys in a secure manner.

digital signature

An asymmetrically encrypted signature whose sole purpose is to authenticate the sender.

directory

A network database that contains a listing of all network resources, such as users, printers, groups, and so on.

directory service

A network service that provides access to a central database of information, which contains detailed information about the resources available on a network.

direct-sequence (DS)

A method of communication between wireless receivers.

direct-sequence spread spectrum (DSSS)

A communications technology that is used to communicate in the 802.11 standard.

disaster recovery

The act of recovering data following a disaster that has destroyed it.

disaster recovery plan

A plan outlining the procedure by which data is recovered after a disaster.

Discretionary Access Control (DAC)

A method of restricting access to objects based on the identity of the subjects or the groups to which they belong.

disk mirroring

Technology that keeps identical copies of data on two disks to prevent the loss of data if one disk faults.

disk striping

Technology that enables writing data to multiple disks simultaneously in small portions called stripes. These stripes maximize use by having all the read/write heads working constantly. Different data is stored on each disk and isn't automatically duplicated (this means disk striping in and of itself doesn't provide fault tolerance).

disk striping with parity

A fault-tolerance solution of writing data across a number of disks and recording the parity on another. In the event any one disk fails, the data on it can be re-created by looking at the remaining data and computing parity to figure out the missing data.

distributed denial of service (DDoS) attack

A derivative of a DoS attack in which multiple hosts in multiple locations all focus on one target to reduce its availability to the public. See denial of service (DoS) attack.

DMZ

See demilitarized zone (DMZ).

DNS

See Domain Name Service (DNS).

DNS server

Any server that performs address resolution from a DNS fully qualified domain name (FQDN) to an IP address. See also Domain Name Service (DNS), Internet Protocol (IP).

DNS zone

An area in the DNS hierarchy that is managed as a single unit. See also Domain Name Service (DNS).

DoD Networking Model

A four-layer conceptual model describing how communications should take place between computer systems. The four layers are Process/Application, Host-to-Host, Internet, and Network Access.

domain

Within the Internet, a group of computers with shared traits and a common IP address set. A domain can also be a group of networked Windows computers that share a single SAM database. See also Security Accounts Manager (SAM).

Domain Name Service (DNS)

The network service used in TCP/IP networks that translates hostnames to IP addresses. See also Transmission Control Protocol/Internet Protocol (TCP/IP).

DoS attack

See denial of service (DoS) attack.

DS

See direct-sequence (DS).

dual-homed host

A host that resides on more than one network and possesses more than one physical network card.

dumb terminal

A keyboard and monitor that send keystrokes to a central processing computer (typically a mainframe or minicomputer) that returns screen displays to the monitor. The unit has no processing power of its own, hence the moniker "dumb."

Dumpster diving

Looking through trash for clues—often in the form of paper scraps—to find users' passwords and other pertinent information.

duplexed hard drives

Two hard drives to which identical information is written simultaneously. A dedicated controller card controls each drive. Used for fault tolerance.

duplicate servers

Two servers that are identical, for use in clustering.

Dynamic Host Configuration Protocol (DHCP)

A protocol used on a TCP/IP network to send client configuration data, including IP address, default gateway, subnet mask, and DNS configuration, to clients. DHCP uses a four-step process: Discover, Offer, Request, and Acknowledgement. See also default gateway, Domain Name Service (DNS), Transmission Control Protocol/Internet Protocol (TCP/IP).

dynamic packet filtering

A type of firewall used to accept or reject packets based on their contents.

dynamic routing

The use of route-discovery protocols to talk to other routers and find out what networks they are attached to. Routers that use dynamic routing send out special packets to request updates from the other routers on the network as well as to send their own updates.

dynamically allocated port

A TCP/IP port that is not constantly used but accessed by an application when needed.