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  what is MAC address
Wed Nov 26, 2008 9:24 pm 

what is MAC address:
The Data Link Layer consists of two sub-layers, the Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a network node gains access to the data and permission to transmit it.

MAC addresses are globally unique addressed that are written into hardware at the time of manufacture. For this reason, MAC addresses are sometimes called Burned In Addresses (BIA).

After manufacture, it is possible in many cases to change the MAC address of a device in software.

MAC addresses are used by many different Layer 2 technologies, including Ethernet, Token Ring, 802.11, Bluetooth, FDDI, ATM, SCSI, and Fibre Channel.

Because MAC addresses originated in the Ethernet specification, the MAC address is sometimes referred to as the Ethernet Address.

A MAC address is 48 bits long. This means that there are 281,474,976,710,656 possible MAC addresses.

A sample MAC address looks like this:

00-0C-F1-56-98-AD

The first three bytes of this address (00-0C-F1) identify the manufacture of this network device, in this case Intel.

Really , What Is a MAC Address?

The MAC address is a unique value associated with a network adapter. MAC addresses are also known as hardware addresses or physical addresses. They uniquely identify an adapter on a LAN.

MAC addresses are 12-digit hexadecimal numbers (48 bits in length). By convention, MAC addresses are usually written in one of the following two formats:

Code:
    MM:MM:MM:SS:SS:SS


    MM-MM-MM-SS-SS-SS


The first half of a MAC address contains the ID number of the adapter manufacturer. These IDs are regulated by an Internet standards body (see sidebar). The second half of a MAC address represents the serial number assigned to the adapter by the manufacturer. In the example,

Code:
    00:A0:C9:14:C8:29


The prefix

Code:
00A0C9


indicates the manufacturer is Intel Corporation.
Why MAC Addresses?
Recall that TCP/IP and other mainstream networking architectures generally adopt the OSI model. In this model, network functionality is subdivided into layers. MAC addresses function at the data link layer (layer 2 in the OSI model). They allow computers to uniquely identify themselves on a network at this relatively low level.

MAC vs. IP Addressing
Whereas MAC addressing works at the data link layer, IP addressing functions at the network layer (layer 3). It's a slight oversimplification, but one can think of IP addressing as supporting the software implementation and MAC addresses as supporting the hardware implementation of the network stack. The MAC address generally remains fixed and follows the network device, but the IP address changes as the network device moves from one network to another.

IP networks maintain a mapping between the IP address of a device and its MAC address. This mapping is known as the ARP cache or ARP table. ARP, the Address Resolution Protocol, supports the logic for obtaining this mapping and keeping the cache up to date.

DHCP also usually relies on MAC addresses to manage the unique assignment of IP addresses to devices.

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