Why is the MAC address 48 -bit?

"MAC address" is an address for specifying the destination for communication with Ethernet (wired LAN) or Wi-Fi (wireless LAN).In addition to PCs and smartphones, MAC addresses are assigned to network equipment such as routers and LAN switches.When communicating in the LAN, the data is delivered using this MAC address as a clue.It plays the role of "address".

When it comes to network addresses, many people may think of an "IP address".Both MAC addresses and IP addresses are common in terms of "address" in the network.The IP address plays the role of an "address" in the vast network of the Internet.On the other hand, the MAC address is used as a relatively narrow network, and in other words, a transmission media (LAN cable, wireless, etc.) directly connected.

I can't say which MAC address or IP address is more important.Without either one, data cannot be delivered to the Internet.In other words, the MAC address and the IP address are complemented.How to complement each other in another time.

The MAC address is a bit of 48 bits.The number of 0 and 1 of the binary number is 48 lined up.Usually, to make it easier for people to understand, divide 48 bits columns into six (called octets in the communication field) and write them in hexadecimal.

For example, the binary number of "010100111011011011111100100100100111111100" will be "52: 3A: 3e: 72: C9: BC" when it is converted to hexadecimal numbers.In addition, the MAC address in the hexadecimal number notification is separated into two octets, but may be used in addition to the colon.

The MAC address is not lined up flat on flat.I have a certain structure.First, there are "I / G bit" and "G / L bit" that define the communication method and the range of use.This is the first bit to be read by the receiving side and the bit to read the second.

The reason why we say such a troublesome way is for Ethernet's unique specifications.In Ethernet, when receiving data (called a frame) on the receiving side, it is taken in octet units, and it is read from the lowest bit.

I / G bit is a bit that determines the communication method."I" means "Individual", and "G" means "Group".If this value is "0", it means that the communication partner is only one individual terminal.This communication method is called "unicast".This is used for normal communication.On the other hand, if this value is "1", the data is sent to multiple communication partners at the same time.There are two types of this method, "multicast", which sends data to specific groups, and "Broadcast", which sends data to all terminals in the LAN.

G / L bit is a bit that determines the range of use."G" means "global", and "l" means "local".In the case of "0", it is a unique address that can be used around the world, and in the case of "1", it is an address used only with closed local networks.The MAC address unique to the equipment is determined to be "0"."1" was assumed to be used for experiments and was rarely used.However, in recent iOS, etc., the MAC address with G / L bit as "1" has been used.

Another big point in the structure of the MAC address is that it is divided into the first half and the second half.In the case of a MAC address used for unicasts, the first 24 -bit is called "OUI (Organizationaly Unique Identifier)", and a unique value is assigned for each device vendor.In the latter 24 bits, each vendor assigns it to its own products.This can avoid duplication of addresses.

In the case of multicast, the first half is determined for each application (protocol).For example, in the case of a protocol called "IP multicast", the first half of the MAC address is defined as "01: 00: 5e".The second half is a form of mapping a part of the IP address.In addition, the MAC address for the broadcast is determined to be "1" for all bit and "FF: ff: ff: ff: ff: ff" if it is a hexadecimal.

In the explanation so far, you may have thought that the MAC address only uses the fixed one, but in fact OS can be generated or users and administrators can change.For example, Windows allows you to change your MAC address by changing the property or registry.The MAC address assigned to the device does not change, but the MAC address actually used in communication is different.

In recent iOS, Android, and Windows, from the viewpoint of privacy enhancement, apart from the MAC address assigned to the device, it has a function to use randomly generated MAC addresses for communication.I will explain this in detail in another time.

This article is a paid members only.Please log in or apply on the next page.

Next page Q: Why is the MAC address 48 -bit?