M

MAC (Media Access Control) address

A six-octet number that uniquely identifies a host on a network. It is a unique number that is burned into the network interface.

MAC layer

In the OSI model, the lower of the two sublayers of the data-link layer. It is defined by the IEEE as being responsible for interaction with the physical layer.

mainframe system

A large computer network in which the central computer handles all the data processing and storage; only the results that are requested are sent to the requesting nodes.

MAN (metropolitan area network)

A network that spans a defined geographical location such as a city or suburb.

master name server

The supplying name server that has authority in a DNS zone.

MAU (media access unit)

A transceiver that is specified in IEEE 802.3. Not to be confused with a Token Ring multistation access unit, which is abbreviated MSAU.

Mb (megabit)

1 million bits. Used to rate transmission transfer speeds.

MB (megabyte)

1 million bytes. Usually refers to file size.

Mbps (megabits per second)

The number of millions of bits that can travel across a given medium in a second. Used as a measurement for the bandwidth of network media.

MDI (medium-dependent interface)

A type of port found on Ethernet networking devices such as hubs and switches in which the wiring is straight through. MDI ports are sometimes referred to as uplink ports and are intended for use as connectivity points to other hubs and switches.

MDI-X (medium-dependent interface crossed)

A type of port found on Ethernet networking devices in which the wiring is crossed so that the transmit line of one device becomes the receive line of the other. MDI-X is used to connect hubs and switches to client computers.

memory address

The label assigned to define the location in memory where information is stored. Usually expressed in binary.

message

A portion of information that is sent from one node to another. Messages are created at the upper layers of the OSI model.

metric

A value that can be assigned to a route to encourage or discourage the use of the route.

See also [cost]

MIB (Management Information Base)

A data set that defines the criteria that can be retrieved and set on a device, using SNMP.

microsegmentation

The process of using switches to divide a network into smaller segments.

microwaves

Very short radio waves that are used to transmit data.

mirroring

A fault-tolerant technique in which an exact duplicate of data on one volume is created on another. Mirroring is defined as RAID 1.

See also [RAID]

modem (modulator-demodulator)

A device used to modulate and demodulate the signals that pass through it. It converts the direct current pulses of the serial digital code from the controller into the analog signals that are compatible with the telephone network.

MSAU (multistation access unit)

A hub that is used in an IBM Token Ring network. It organizes the connected nodes into an internal ring and uses the RI and RO connectors to expand to other MSAUs on the network. Sometimes referred to as MAU.

MTBF (mean time between failure)

The amount of time, normally expressed in hours, that represents the average amount of time a component will function before it fails.

MTTF (mean time to fix)

The amount of time it normally takes to fix a problem or swap out a component.

multicast

A single-packet transmission from one sender to a specific group of destination nodes.

multihomed

A term used to refer to a device that has more than one network interface.

multiplatform

A term used to refer to a programming language, technology, or protocol that runs on different types of CPUs or operating systems.

multiplexing

A method of transmitting multiple logical signals across the same channel at the same time.

multiprocessor

A term that refers to the use of multiple processors in a single system.

multitasking

The running of several programs simultaneously. In actuality, during multitasking the processor is sharing its time between the programs, and it only appears as if they are running concurrently.

multithreading

A form of multitasking in which the different tasks that appear to be running concurrently are coming from the same application rather than from different applications.