Media Access Control. Lower of the two sublayers of the data link layer defined by the IEEE. The MAC sublayer handles access to shared media, such as whether token passing or contention will be used. See also data link layer and LLC.

MAC address

Standardized data link layer address that is required for every port or device that connects to a LAN. Other devices in the network use these addresses to locate specific ports in the network and to create and update routing tables and data structures. MAC addresses are 6 bytes long and are controlled by the IEEE. Also known as a hardware address, a MAC-layer address, or a physical address. Compare with network address.

MAC address learning

Service that characterizes a learning bridge, in which the source MAC address of each received packet is stored so that future packets destined for that address can be forwarded only to the bridge interface on which that address is located. Packets destined for unrecognized addresses are forwarded out every bridge interface. This scheme helps minimize traffic on the attached LANs. MAC address learning is defined in the IEEE 802.1 standard. See also learning bridge and MAC address.


Network layer protocol that encapsulates IP packets in DDS or transmission over AppleTalk. MacIP also provides proxy ARP services.

MAC-layer address

See MAC address.

Main cross connect

See MCC.

Main distribution facility

See MDF.

Maintenance Operation Protocol

See MOP.


metropolitan-area network. Network that spans a metropolitan area. Generally, a MAN spans a larger geographic area than a LAN, but a smaller geographic area than a WAN. Compare with LAN and WAN.

managed object

In network management, a network device that can be managed by a network management protocol.

Management Information Base

See MIB.

management services

SNA functions distributed among network components to manage and control an SNA network.

Manchester encoding

Digital coding scheme, used by IEEE 802.3 and Ethernet, in which a mid-bit-time transition is used for clocking, and a 1 is denoted by a high level during the first half of the bit time.

Manufacturing Automation Protocol

See MAP.


Manufacturing Automation Protocol. Network architecture created by General Motors to satisfy the specific needs of the factory floor. MAP specifies a token-passing LAN similar to IEEE 802.4. See also IEEE 802.4.


See address mask and subnet mask.

master management agent

See MMA.


media attachment unit. Device used in Ethernet and IEEE 802.3 networks that provides the interface between the AUI port of a station and the common medium of the Ethernet. The MAU, which can be built into a station or can be a separate device, performs physical layer functions including the conversion of digital data from the Ethernet interface, collision detection, and injection of bits onto the network. Sometimes referred to as a media access unit, also abbreviated MAU, or as a transceiver. In Token Ring, a MAU is known as a multistation access unit and is usually abbreviated MSAU to avoid confusion. See also AUI and MSAU.

maximum burst

Specifies the largest burst of data above the insured rate that will be allowed temporarily on an ATM PVC, but will not be dropped at the edge by the traffic policing function, even if it exceeds the maximum rate. This amount of traffic will be allowed only temporarily; on average, the traffic source needs to be within the maximum rate. Specified in bytes or cells. Compare with insured burst. See also maximum rate.

maximum rate

Maximum total data throughput allowed on a given virtual circuit, equal to the sum of the insured and uninsured traffic from the traffic source. The uninsured data might be dropped if the network becomes congested. The maximum rate, which cannot exceed the media rate, represents the highest data throughput the virtual circuit will ever deliver, measured in bits or cells per second. Compare with excess rate and insured rate. See also maximum burst.

maximum transmission unit

See MTU.






multicast backbone. The multicast backbone of the Internet. MBONE is a virtual multicast network composed of multicast LANs and the point-to-point tunnels that interconnect them.


megabits per second.


micro channel architecture. Bus interface commonly used in PCs and some UNIX workstations and servers.


main cross-connect. Wiring closet that serves as the most central point in a star topology and where LAN backbone cabling connects to the Internet.


Multiport Communications Interface. Card on the AGS+ that provides two Ethernet interfaces and up to two synchronous serial interfaces. The MCI processes packets rapidly, without the interframe delays typical of other Ethernet interfaces.


minimum cell rate. Parameter defined by the ATM Forum for ATM traffic management. MCR is defined only for ABR transmissions, and specifies the minimum value for the ACR. See also ABR (available bit rate), ACR, and PCR.


Message Digest 5. Algorithm used for message authentication in SNMP v.2. MD5 verifies the integrity of the communication, authenticates the origin, and checks for timeliness. See also SNMP2.


Main distribution facility. Primary communications room for a building. Central point of a star networking topology where patch panels, hub, and router are located.


Plural of medium. The various physical environments through which transmission signals pass. Common network media include twisted-pair, coaxial and fiber-optic cable, and the atmosphere (through which microwave, laser, and infrared transmission occurs). Sometimes called physical media.

Media Access Control

See MAC.

Media Access Control Address

See MAC address.

media access unit

See MAU.

media attachment unit

See MAU.

media interface connector

See MIC.

media rate

Maximum traffic throughput for a particular media type.


See media.

medium-speed line card

See MSC.


Abbreviated Mb.

megabits per second

Abbreviated Mbps.


Abbreviated MB.


Network topology in which devices are organized in a manageable, segmented manner with many, often redundant, interconnections strategically placed between network nodes. See also full mesh and partial mesh.


Application layer (Layer 7) logical grouping of information, often composed of a number of lower-layer logical groupings such as packets. The terms datagram, frame, packet, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

message handling system

See MHS.

Message Digest 5

See MD5.

Message Queuing Interface

See MQI.

message switching

Switching technique involving transmission of messages from node to node through a network. The message is stored at each node until such time as a forwarding path is available. Contrast with circuit switching and packet switching.

message unit

Unit of data processed by any network layer.

Metal oxide varister

See MOV.


Process running at the ATM layer that manages signaling types and virtual circuits.


See traffic shaping.


See routing metric.

metropolitan-area network

See MAN.


Cisco midrange multiprotocol router designed for medium to small regional and district environments. The MGS is a 4-slot router that can handle up to 11 interfaces of different types.


Main distribution facility. Primary communications room for a building. Central point of a star networking topology where patch panels, hub, and router are located. See also IDF


message handling system. ITU-T X.400 recommendations that provide message handling services for communications between distributed applications. NetWare MHS is a different (though similar) entity that also provides message-handling services. See also IFIP.


Management Information Base. Database of network management information that is used and maintained by a network management protocol such as SNMP or CMIP. The value of a MIB object can be changed or retrieved using SNMP or CMIP commands. MIB objects are organized in a tree structure that includes public (standard) and private (proprietary) branches.

MIB collection

Polling technique used by the SNMP protocol to gather information needed to monitor the network.

MIB reporting

Technique used by the CMIP protocol to obtain information needed to monitor the network. It is dependent upon network devices to initiate reports regarding their status to a central monitoring station on the network.


media interface connector. FDDI de facto standard connector.

micro channel architecture

See MCA.


Translation layer between machine instructions and the elementary operations of a computer. Microcode is stored in ROM and allows the addition of new machine instructions without requiring that they be designed into electronic circuits when new instructions are needed.


Division of a network into smaller segments, usually with the intention of increasing aggregate bandwidth to network devices.


Unit of measure equal to one millionth of a meter or one thousandth of a millimeter. Sometimes the symbol is used instead of the word micron.


Electromagnetic waves in the range 1 to 30 GHz. Microwave-based networks are an evolving technology gaining favor due to high bandwidth and relatively low cost.


Broadband cable system in which the available frequencies are split into two groups: one for transmission and one for reception.

Military Network


millions of instructions per second

See mips.


Military Network. Unclassified portion of the DDN. Operated and maintained by the DISA. See also DDN and DISA.

minimum cell rate

See MCR.


MultiChannel Interface Processor. Interface processor on the Cisco 7000 series routers that provides up to two channelized T1 or E1 connections via serial cables to a CSU. The two controllers on the MIP can each provide up to 24 T1 or 30 E1 channel-groups, with each channel-group presented to the system as a serial interface that can be configured individually.


millions of instructions per second. Number of instructions executed by a processor per second.


master management agent. SNMP agent that runs on the NP of a LightStream 2020 ATM switch. MMA translates between an external network manager using SNMP and the internal switch management mechanisms.


modulator-demodulator. Device that converts digital and analog signals. At the source, a modem converts digital signals to a form suitable for transmission over analog communication facilities. At the destination, the analog signals are returned to their digital form. Modems allow data to be transmitted over voice-grade telephone lines.

modem eliminator

Device allowing connection of two DTE devices without modems.


Process by which the characteristics of electrical signals are transformed to represent information. Types of modulation include AM, FM, and PAM. See also AM, FM, and PAM.


See modem.


Management tool on the LightStream 2020 ATM switch that allows a user to examine individual nodes in the network and learn the status of interface modules and power supplies. The monitor is an HP OpenView-based application that runs on an NMS.

monomode fiber

See single-mode fiber.


Maintenance Operation Protocol. Digital Equipment Corporation protocol, a subset of which is supported by Cisco, that provides a way to perform primitive maintenance operations on DECnet systems. For example, MOP can be used to download a system image to a diskless station.


Public-domain WWW browser, developed at the National Center for Supercomputing Applications (NCSA). See also WWW browser.


Multicast OSPF. Intradomain multicast routing protocol used in OSPF networks. Extensions are applied to the base OSPF unicast protocol to support IP multicast routing.


metal oxide varistor. Voltage clamping surge suppressor capable of absorbing large currents without damage.


Message Queuing Interface. International standard API that provides functionality similar to that of the RPC interface. In contrast to RPC, MQI is implemented strictly at the application layer. See also RPC.


multistation access unit. Wiring concentrator to which all end stations in a Token Ring network connect. The MSAU provides an interface between these devices and the Token Ring interface of, for example, a Cisco 7000 TRIP. Sometimes abbreviated MAU.


medium-speed line card. Card on the LightStream 2020 ATM switch that can be configured as an edge or a trunk card. The MSC, in conjunction with an access card, supports two trunk or edge (UNI) ports at data rates up to T3 or E3.


maximum transmission unit. Maximum packet size, in bytes, that a particular interface can handle.


North American companding standard used in conversion between analog and digital signals in PCM systems. Similar to the European alaw. See also a-law and companding.

multiaccess network

Network that allows multiple devices to connect and communicate simultaneously.


Single packets copied by the network and sent to a specific subset of network addresses. These addresses are specified in the destination address field. Compare with broadcast and unicast.

multicast address

Single address that refers to multiple network devices. Synonymous with group address. Compare with broadcast address and unicast address. See also multicast.

multicast backbone


multicast group

Dynamically determined group of IP hosts identified by a single IP multicast address.

Multicast OSPF


multicast router

Router used to send IGMP query messages on their attached local networks. Host members of a multicast group respond to a query by sending IGMP reports noting the multicast groups to which they belong. The multicast router takes responsibility for forwarding multicast datagrams from one multicast group to all other networks that have members in the group. See also IGMP.

multicast server

Establishes a one-to-many connection to each device in a VLAN, thus establishing a broadcast domain for each VLAN segment. The multicast server forwards incoming broadcasts only to the multicast address that maps to the broadcast address.

MultiChannel Interface Processor

See MIP.

multidrop line

Communications line having multiple cable access points. Sometimes called a multipoint line.

multihomed host

Host attached to multiple physical network segments in an OSI CLNS network.


Addressing scheme in IS-IS routing that supports assignment of multiple area addresses.

multilayer switch

Switch that filters and forwards packets based on MAC addresses and network addresses. A subset of LAN switch. The Catalyst 5000 is an example of a multilayer switch. Compare with LAN switch.

multimode fiber

Optical fiber supporting propagation of multiple frequencies of light. See also single-mode fiber.

multiple domain network

SNA network with multiple SSCPs. See also SSCP.


Scheme that allows multiple logical signals to be transmitted simultaneously across a single physical channel. Compare with demultiplexing.

multipoint line

See multidrop line.

Multiport Communications Interface

See MCI.

multistation access unit


multivendor network

Network using equipment from more than one vendor. Multivendor networks pose many more compatibility problems than single-vendor networks. Compare with single-vendor network.