M

MAC (Machine Authentication Code)

A way to check the integrity of information transmitted over, or stored on, an unreliable medium, based on a secret key. Typically, MACs are used between two parties who share a secret key, in order to validate the information transmitted between the two parties. key-dependent, one-way hash function, requiring the use of the identical key to verify the hash.
See also HMAC.

MAC address (Media Access Control)

One of the two addresses every networked computer has (the other being an IP address), a Media Access Control address is a unique 48-bit identifier usually written as 12 hexadecimal characters grouped in pairs (e. g., 00-00-0c-34-11-4e). This address is usually hard-coded into a Network Interface Card (NIC) by its manufacturer, and does not change. It is the physical address of a data device, and is used as an aid for routers trying to locate machines on large networks.

mail server

Refers to both the application and the physical machine tasked with routing incoming and outgoing electronic mail.

MD2 (Message Digest 2)

128-bit, one-way hash function that is dependent on a random permutation of bytes. MD2 is considered very secure, but takes a long time to compute, and therefore is rarely used.
See also message digest.

MD4 (Message Digest 4)

A 128-bit, one-way hash function that uses a simple set of bit manipulations on 32-bit operands, developed as a weaker but faster alternative to MD2.
See also message digest.

MD5 (Message Digest 5)

A more secure, more complex version of MD4, but still a 128-bit, one-way hash function. Although now widely used, MD5 contains a few flaws discovered in 1996 making it slightly weaker, so it is gradually falling out of favor in deference to another message digest function known as SHA-1.
See also message digest.

message digest

A mathematical function used in encryption to distill the information contained in a file into a single large number, typically between 128 and 256 bits in length. Message digests are also known as one-way hash functions because they produce results where it is mathematically infeasible to try to calculate the original message by computing backwards from the result. Message digest functions are designed so that a change to a single character in the message will cause the message to result in a very different message digest number. Many different message digest functions have been proposed and are now in use; most are considered highly resistant to attack.

MIME (Multipurpose Internet Mail Extensions)

A specification for formatting non-ASCII messages so that they can be sent over the Internet. Many e-mail clients now support MIME, which enables them to send and receive graphics, audio, and video files via the Internet mail system. In addition, MIME supports messages in character sets other than ASCII.

MIME type

Though MIME was developed for email (SMTP), it proved so useful that other application protocols also adopted it. For example, HTTP uses MIME headers to indicate what sort of data is being transferred. RFC 2046 defines how MIME can handle various media types, such as image, audio, video, and application. MIME content types are expressed as a type and a subtype, separated by a slash. (Example: image/jpeg).

modem

A shortened version of "modulator/demodulator," this is the word for a communications device that sends computer transmissions over a standard telephone line.

motherboard

The main printed circuit board in a computer, which contains sockets that accept additional boards (daughterboards).

MSDUN

An abbreviation for Microsoft Dial-Up Networking, required for remote user VPN.

multiple network configuration

See routed mode.