Multicast MAC Address _ turn

Source: Internet
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Turn from: Multicast MAC address

The MAC address is a 48bit (6-byte hexadecimal number) address used by the Ethernet two layer to identify the device location. The MAC address is divided into two parts, the first 24 bits being the organization's unique identifier (OUI, organizationally unique identifier), and the last 24 bits being assigned by the vendor itself.

MAC addresses have unicast, multicast, broadcast points. Unicast addresses (unicast address) indicate that a single device, node, multicast address, or multicast address (multicast addresses, group address) represents a set of devices, nodes, broadcast addresses (broadcast address) is a special case of multicast, Represents all addresses, expressed in full F: ff-ff-ff-ff-ff-ff. Of course, the three-tier IP address also has unicast, multicast, broadcast points.

The MAC address of the

48bit is typically expressed in 6-byte hexadecimal notation, such as xx-xx-xx-xx-xx. IEEE 802.3 stipulates that: Ethernet 48bit (modified to the first bit in 2012-04-11) is used to indicate whether this address is a multicast address or a unicast address . If this bit is 0, this MAC address is a unicast address, and if this is 1, this MAC address is a multicast address. See IEEE 802.3 3.2.3 Address fields: "The first bit (LSB) shall is used in the Destination address field as an address Type de Signation bit to identify the Destination address either as an individual or as a group address. If This bit was 0, it shall indicate that address field contains an individual address. If This bit is 1, it shall indicate so the Address field contains a group address that identifies none, one or more, or All of the stations connected to the LAN. The Source Address field, the first bit is reserved and set to 0. "

On the Ethernet line, the "Big Endian" byte sequence is transmitted (that is, the highest byte is transmitted first, refer to the relevant document for the byte order), and the bit sequence is "Little Endian" (that is, the lowest bit is transmitted first). a hexadecimal representation of the MAC address 01-80-c2-00-00-00, the bit order to transmit is: 0000 0000 0001 01000011 0000 0000 0000 0000 0000 0000, as shown in the figure below (transfer from the far left):

Note 47bit on the figure (modified to the second bit in 2012-04-11), this one indicates whether the MAC address is a global unique address or a local address, 0 represents a global unique address, and 1 represents a local unique address. This bit is also called the G/L bit.

for a cured MAC address on a network device, because it uniquely identifies the device, it can only be a unicast address, which is the 48th bit of the source address in the Mac frame (modified to the first bit in 2012-04-11) can only be 0.

We often say that there are 2 of the 48-time MAC address can be used for network devices, these addresses can be more than the earth on every grain of sand to allocate an address, in fact, this number is discounted, because MAC address, although there are so many, But really used on the network card and only 2 of the world only 46 square: 48bit must be 0, 47bit must be 0.

This also leads to an interesting phenomenon: to find a PC, look at its network card address, the 1th byte hexadecimal number is generally a multiple of 4; Check the IEEE Assigned oui ( Oui/oui.txt), the hexadecimal number of the 1th byte is also generally a multiple of 4 (early Ethernet does not have the concept of local address, so the allocation of oui inside the G/L bit may also be 1), this case is not a multiple of 4, but certainly a multiple of 2, because the 48th bit can only be 0.

About the multicast address, there is a misunderstanding: the 1th byte of the MAC address must be 0x01 to represent the multicast address, even in the TCP/IP details said (see the Chinese version of the first paragraph 12.4.2). The IEEE 802.3 has made it clear that the multicast address is represented as long as 48bit is 1, so no matter whether the 1th byte of the MAC address is 0x01, 0xc1, or 0x33, the MAC address is the multicast address (the IPV6 corresponding two-tier multicast address that starts with 0x33). The reason for this misunderstanding is that, so far, the 1th byte of most multicast MAC addresses is 0x01. Such as:

01-80-C2-00-00-00 (STP protocol use)

01-80-C2-00-00-01 (use of the pause frame for MAC control)

01-80-c2-00-00-02 (Slow protocol:802.3ah OAM/LACP protocol all with this address, this address is a story, how many software processing this address will be a problem ah! )

01-00-5E-XX-XX-XX (two layer multicast address corresponding to IP multicast address).

For a complete list, see

The reason most multicast addresses start with 01-80-C2 and 01-00-5E is because the protocols that use these multicast addresses are the names of the leading eldest brother IEEE and IANA, and their oui are 00-80-c2 and 00-00-5e respectively, Become the multicast address is 01-80-C2 and 01-00-5e, of course, in addition to take the lead eldest brother grabbed these multicast address, there is 01-00-0c-cc-cc-cc such address, this address is Cisco occupied, Cisco Oui is 00-00-0c.

Multicast MAC Address _ turn

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