★ MAC address definition
MAC (Media Access Control) address, also known as the hardware address, the length is 48 bits (6 bytes), by the hexadecimal number, divided into the first 24 and 24:
The first 24 bits are called Organizationally Unique Identifier (OUI), which is assigned by the IEEE registry to different manufacturers to distinguish between different manufacturers.
The last 24 bits are assigned by the manufacturer themselves, called the extension identifier. The same manufacturer of the network card MAC address after 24 is different.
The MAC address corresponds to the second layer data link layer of the OSI reference model. The switch at the data link layer maintains the computer MAC address and the database of its own port. The switch according to the "destination MAC address" field in the received data frame To forward data frames.
The first byte of the first 8Bit (00-50-BA -... in the corresponding 00000000-01010000-10111010 -..., bold font Bit) identifies whether this address is a multicast address or a unicast address The This is determined by the Ethernet transmission protocol high byte first pass, but within each byte of the low first pass the characteristics of the decision, see IEEE 802.3 3.2.3 Address fields: "The first bit (LSB) shall be used in the Destination Address field as an address type designation bit to identify the Destination Address either as an individual or as a group address. If this bit is 0, it shall indicate that the address field contains an individual address. If this bit is 1, it shall Indicate that the address field contains a group address that indicates none, one or more, or all of the stations connected to the LAN. In the Source Address field, the first bit is reserved and set to 0. " In fact, the order of this transmission is 000000000000101001011101 ... "The first bit (LSB)" is the preamble of the first 8Bit.
The physical address of the network card is usually burned by the card manufacturer's EPROM (a flash chip, usually through the program erase), it stores the data is really transmitted to identify the data sent to the computer and receive data host address.
In other words, in the physical layer of the underlying network transmission process, through the physical address to identify the host, it must be the world's only. For example, the famous Ethernet card, its physical address is 48bit (bit) of the integer, such as: 44-45-53-54-00-00, machine-readable way into the host interface. The Ethernet address management authority (IEEE) (IEEE: Institute of Electrical and Electronics Engineers) divides the Ethernet address, which is a different combination of 48 bits, into several independent continuous address groups that produce Ethernet network card manufacturers to buy one of them, the specific production, one by one will be the only address given to the Ethernet card.