Shulou(Shulou.com)06/01 Report--

Ethernet addressing

In the data link layer, data frames usually rely on MAC addresses for data exchange, which requires global uniqueness like public network IP addresses, so that each host can be identified. So how does the MAC address do this? What is its format?

MAC address, English full name Medium Access Control, literally translates as media access control, it is usually solidified on each Ethernet network card (NIC,Network Interface Card). The MAC (hardware) address is 48 bits (6 bytes) long and is in hexadecimal format. The following figure illustrates the 48-bit MAC address and its components.

Example: 00-01-6C-06-A6-29 or 00:01:6C:06:A6:29

The organization unique identifier (OUI) is assigned to the manufacturer by the IEEE (Institute of Electrical and Electronic Engineers) and contains 24 digits. Manufacturers use the remaining 24 bits (EUI, extended unique identifier) to assign a globally unique global management address for each network card they produce. Generally speaking, large manufacturers will purchase multiple OUI.

The I/G=0 G (Individual/Group) bit is the MAC address of a device, that is, a unicast address, and if I/G=1, it is a multicast address (multicast + broadcast = multicast).

The Global/Local L (Global/Local, where U represents Universal) bit, if G/L=0, is the global management address assigned by IEEE; if G/L=1, it is the local management address, which is specified by the network administrator in order to strengthen his network management.

At present, there are two theories, or two formats, about the position of the G and G / L bits of Ipaw.

For data transmission, the data is transmitted bit by bit in each byte, and one byte is transferred to the next byte.

First, the high bit of each byte is the first, the low bit is the last, the I / G bit and Ghand L are in the lowest bit and the second lowest bit in the byte, the highest bit is sent first, and it is still the highest bit when it reaches the opposite end.

Second, the low bit of each byte comes first, the high bit comes after, the I / G bit and Ghand L are in the lowest bit and the second lowest bit in the byte, respectively, and the lowest bit is sent first, which is still the lowest bit when it reaches the opposite end.

The two seem to be different, but the results are still the same. If you still don't understand, you can draw a picture on draft paper, which is not difficult to understand.

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