What is a MAC Address
What is a MAC address?
The MAC address (MAC address in English, where MAC stands for Media Access Control), also known as physical address, ethernet address or LAN address is a code of 48 bits (6 bytes) uniquely assigned to each Ethernet network adapter produced in the world. It is essentially a name for a particular network device: for example, two network cards on two different computers will have two different names (and therefore different MAC address), will have different names as a wireless Ethernet card and a card placed in the same computer.
MAC is an acronym for Media Access Control and is used for access to the physical layer from the data link layer according to ISO / OSI.
Most layer 2 protocol uses one of three numbering spaces governed by the IEEE: MAC-48, EUI-48, and EUI-64. They are designed to be globally unique, even if not all the communication protocols require this feature. The IEEE claims trademarks on the names “EUI-48″ and “EUI-64″. (The symbol “EUI” stands for Extended Unique Identifier.)
The task of conversion from Layer 3 addresses how the Internet Protocol addresses to MAC layer 2 is commonly delegated at ARPA. In broadcast networks such as Ethernet, the MAC address uniquely identifies each host and allows you to mark the frame as targeted at specific hosts. It is therefore within the Data Link Layer, the basis on which rest the protocols above the OSI model.
MAC Address format
The original IEEE 802 MAC address, now officially called “MAC-48″, comes from the specific Dell ethernet. As originally designed the Ethernet who had the foresight to use a 48-bit address space, we now have the potential of well 248 (ie, 281,474,976,710,656) possible MAC addresses, a number that is virtually impossible to achieve before the ethernet cards change standard.
In this format (the currently most common), the 48-bit code are divided into 12 hex digits: the first 6 digits identify the manufacturer of the network while the later are the serial number of the card. The MAC address is usually written in 6 octets separated by a hyphen (for example, 00-50-FC-A0-67-2C) and the first 3 octets are called the OUI (Organizationally Unique Identifier). For such addresses usually prefer the hexadecimal notation also to differentiate it from IP addresses that use the decimal notation.
The three numbering systems use the same format and differ only in the length of the identifier. Addresses can either be “universally administered address” and “locally administered addresses.”
Universally, an administered address is assigned to the device by its manufacturer and is sometimes called “burned-in addresses”. The first three octets (in transmission order) identify the organization or producer that issued the identifier and represent the Organizationally Unique Identifier (OUI). The next three (MAC-48 and EUI-48) or five (EUI-64) octets are assigned by the producer only has to respect the constraint of uniqueness. The IEEE requires that the space MAC-48 is not exhausted before the year 2100, while not expected to exhaust the EUI-64 addresses in a reasonably near future.
Locally administered addresses are assigned to a component by a network administrator, overriding the burned-in address. Locally administered addresses do not contain the octet OUI, and the bit is set to 1 indicates global scope and if it is set to 0 indicates local scope
The Universally administered and locally administered addresses are distinguished by means of setting the second least significant bit of the most significant byte of the address. If the bit is 0, the address is universally or locally administered local scope, and if vice versa is a 1, it is locally or globally administered global scope. This bit is 0 in all the OUI. For example, 02-00-00-00-00-01 is a locally administered address.
MAC addresses-48 and EUI-48 are usually represented in hexadecimal format, each octet separated by a dash or colon. An example of a MAC-48 address is “00-08-74-4C-7F-1D”. Comparing the first three octets with the IEEE OUI assignments, one can see how it belongs to Dell Corporation, and the last three octets represent the serial number assigned to the component manufacturer.
Each card has a unique address for the first 24 bits are the manufacturer and the subsequent identification of the card. In this way, each manufacturer has available 224 addresses, so it can produce more than 16 million cards, if a manufacturer produces less, addresses (48 bit) is not assigned are lost, they can not be used by other manufacturers.
It is understandable, therefore, as the MAC address does not change if you move a network card from one LAN to another, while it can change the IP address.
Conversion between MAC address and IP address is by some protocols, the best known is ARP.
Study: From Wikipedia, the free encyclopedia. The text is available under the Creative Commons.
