Table of Contents




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, MAC-layer address, and 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 DDP packets for transmission over AppleTalk. MacIP also provides proxy ARP services. See also DDP and proxy ARP.

MAC-layer address

See MAC address.


metropolitan access exchange. One of a number of Internet exchange points. Examples include MAE West and MAE East. See also CIX, FIX, and GIX.

mail bridge

Mail gateway that forwards e-mail between two or more networks while ensuring that the messages it forwards meet certain administrative criteria. A mail bridge is simply a specialized form of mail gateway that enforces an administrative policy with regard to what mail it forwards.

mail gateway

Machine that connects two or more electronic mail systems (especially dissimilar mail systems on two different networks) and transfers messages between them. Sometimes the mapping and translation can be quite complex, and generally it requires a store-and-forward scheme whereby the message is received from one system completely before it is transmitted to the next system after suitable translations.

mail exchange record

See MX record.

mail exploder

Part of an e-mail delivery system that allows a message to be delivered to a list of addressees. Mail exploders are used to implement mailing lists. Users send messages to a single address (for example,, and the mail exploder takes care of delivery to the individual mailboxes in the list.

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 meet the specific needs of the factory floor. MAP specifies a token-passing LAN similar to IEEE 802.4. See also IEEE 802.4.


Multicast Address Resolution Server. Mechanism for supporting IP multicast. A MARS serves a group of nodes (known as a cluster); each node in the cluster is configured with the ATM address of the MARS. The MARS supports multicast through multicast messages of overlaid point-to-multipoint connections or through multicast servers.


Humorous term applied to packets that turn up unexpectedly on the wrong network because of bogus routing entries. Also used as a name for a packet that has an altogether bogus (nonregistered or ill-formed) Internet address.


See address mask and subnet mask.


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.


megabyte. Approximately 1,000,000 bytes.


megabit. Approximately 1,000,000 bits.


maximum burst size. In an ATM signaling message, burst tolerance is conveyed through the MBS, which is coded as a number of cells. The burst tolerance together with the SCR and the GCRA determine the MBS that can be transmitted at the peak rate and still be in conformance with the GCRA. See also SCP and GCRA.


multicast backbone. 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.


maximum cell delay variation. In an ATM network, the maximum two-point CDV objective across a link or node for the specified service category. One of four link metrics exchanged using PTSPs to determine the available resources of an ATM network. There is one MCDV value for each traffic class. See also CDV and PTSP.


maximum cell loss ratio. In an ATM network, the maximum ratio of cells that do not successfully transit a link or node compared with the total number of cells that arrive at the link or node. One of four link metrics exchanged using PTSPs to determine the available resources of an ATM network. The MCLR applies to cells in the CBR and VBR traffic classes whose CLP bit is set to zero. See also CBR, CLP, PTSP, and VBR.


Multimedia Cable Network System Partners Ltd. Consortium of cable companies providing service to the majority of homes in the United States and Canada. This consortium drives a standard with the goal of having interoperable cable modems.


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), ACOM, and PCR.


maximum cell transfer delay. In an ATM network, the sum of the MCDV and the fixed delay component across the link or node. One of four link metrics exchanged using PTSPs to determine the available resources of an ATM network. There is one MCTD value for each traffic class. See also MCDV and PTSP.


mediation device. Device that provides protocol translation and concentration of telemetry information originating from multiple network elements and transport to an OSS. See also OSS.


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.


The TransPath Message Definition Language. High-level language used to specify protocols and protocol conversion operations on the TransPath.


message disposition notification. Message returned to the originator of an e-mail message indicating that the e-mail message has been opened. Specifications for MDN are described in RFC 2298.


Plural of medium. 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 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.


Abbreviated Mb. Approximately 1,000,000 bits.

megabits per second

Abbreviated Mbps.


Abbreviated MB. Approximately 1,000,000 bytes.


Mercury Exchange Limited (MEL) Channel Associated Signaling. A voice signaling protocol used primarily in the United Kingdom.


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.


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


See traffic shaping.


See routing metric.

metropolitan-area network

See MAN.


multiflex trunk module.


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, usually through a GUI network management system. MIB objects are organized in a tree structure that includes public (standard) and private (proprietary) branches.


media interface connector. FDDI de facto standard connector.


Modem ISDN channel aggregation. Modem module and card used in the Cisco AS5300 universal access servers. A MICA modem provides an interface between an incoming or outgoing digital call and an Integrated Services Digital Network (ISDN) telephone line; the call does not have to be converted to analog, as it does with a conventional modem and an analog telephone line. Each line can accommodate, or aggregate, up to 24 (T1) or 30 (E1)calls.


Device that prevents data frequencies (intended for a data device such as a router) from traveling over the telephone line and interfering with telephone calls.


message identifier. In ATM, used to identify ATM cells that carry segments from the same higher-layer packet.

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.


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.

mid-level network

mid-level networks. Makes up the second level of the Internet hierarchy. They are the transit networks that connect the stub networks to the backbone networks. Also referred to as regionals.


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


media independent interface. Standard specification for the interface between network controller chips and their associated media interface chip(s). The MII automatically senses 10- and 100-MHz Ethernet speeds.

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.


Multipurpose Internet Mail Extension. MIME. Sstandard for transmitting non-text data (or data that cannot be represented in plain ASCII code) in Internet mail, such as binary, foreign language text (such as Russian or Chinese), audio, or video data. MIME is defined in RFC 2045.

minimum cell rate

See MCR.


See MIP (MultiChannel Interface Processor) in the "Cisco Systems Terms and Acronyms" section.


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


multiservice interchange.


Multilink PPP. Method of splitting, recombining, and sequencing datagrams across multiple logical data links.


multimode fiber.


TransPath Man-Machine Language.


Multimedia Mail over IP.

MMoIP dial peer

Multimedia Mail over Internet Protocol dial peer. Dial peer specific to Store and Forward Fax. The MMoIP dial peer is the vehicle you use to assign particular line characteristics (such as a destination telephone number) to the connection between the Cisco router or access server and the SMTP mail server during on-ramp faxing.


Multichassis Multilink PPP. Extends MLP support across multiple routers and access servers. MMP enables multiple routers and access servers to operate as a single, large dial-up pool, with a single network address and ISDN access number. MMP correctly handles packet fragmenting and reassembly when a user connection is split between two physical access devices.


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.

monomode fiber

See single-mode fiber.


Maintenance Operation Protocol. Digital Equipment Corporation protocol 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 NCSA. See also 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.


Motion Picture Experts Group. Standard for compressing video. MPEG1 is a bit stream standard for compressed video and audio optimized to fit into a bandwidth of 1.5 Mbps. MPEG2 is intended for higher quality video-on-demand applications and runs at data rates between 4 and 9 Mbps. MPEG4 is a low-bit-rate compression algorithm intended for 64-kbps connections.


Multiprotocol Label Switching. Emerging industry standard upon which tag switching is based.


Multiprotocol over ATM. ATM Forum standardization effort specifying how existing and future network-layer protocols such as IP, IPv6, AppleTalk, and IPX run over an ATM network with directly attached hosts, routers, and multilayer LAN switches.


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 a router. Sometimes abbreviated MAU.


multiple service operator. Cable service provider that also provides other services such as data and/or voice telephony.


1. Message Transfer Agent. OSI application process used to store and forward messages in the X.400 Message Handling System. Equivalent to Internet mail agent.

2. Mail Transfer Agent. Software that implements SMTP and provides storage for mail messages to be forwarded or delivered to a local user. MTAs implement SMTP (RFC 821).


meantime between failure.


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


multi-user dungeon. Adventure, role playing games, or simulations played on the Internet. Players interact in real time and can change the "world" in the game as they play it. Most MUDs are based on the Telnet protocol.


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

VCC set up by the BUS to the LEC as a leaf in a point-to-multipoint connection. See also BUS, LEC (LAN Emulation Client), and VCC.

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 send VCC

In an ATM network, a bi-directional point-to-point VCC set up by a LEC to a BUS. One of three data connections defined by Phase 1 LANE. Compare with control distribute VCC and control direct VCC. See also BUS, LEC (LAN Emulation Client), and VCC.

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 in the "Cisco Systems Terms and Acronyms" section.

multidrop line

Communications line with 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. 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.


1. Line or channel connecting three or more different service points.

2. Circuit that has points served by three or more switches. Single communications channel (typically a leased telephone circuit) to which two or more stations or logical units are attached although only one can transmit at a time. Such arrangements usually require a polling mechanism under the control of a master station to ensure that only one device transmits at a time.

multipoint control unit

Endpoint on the LAN that provides the capability for three or more terminals and gateways to participate in a multipoint conference.

multipoint line

See multidrop line.

Multiprotocol over ATM


Multipurpose Internet Mail Extension


multistation access unit


multi-user dungeon

See MUD.

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.


multiplexing device. Combines multiple signals for transmission over a single line. The signals are demultiplexed, or separated, at the receiving end.

MX record

mail exchange record. DNS resource record type indicating which host can handle e-mail for a particular domain.

Posted: Tue Sep 21 15:19:51 PDT 1999
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