Detailed description of multicast addresses

IANA has allocated the Class D address space to the IP multicast address. the address of the Class D space is located in the first four digits of the first byte and identified by the binary value 1110. therefore, the multicast address range is: 224.0.0.0 to 239.20.00000000d Class address: www.2cto.com byte 1 byte 2 byte 3 byte 41110 xxxx xxxxxxxx

The principle is as follows: the address of the space is expressed in binary and the first four digits of the first eight digits are represented in 1110. 1110xxxx. xxxxxxxx. xxxxxxxx. some common local link multicast addresses are given below xxxxxxxx: 224.0.0.1 all hosts 224.0.0.2 all multicast routers 224.0.0.3 are not allocated 224.0.0.4 DVMRP router 224.0.0.5 OSPF router 224.0.0.6 OSPF designated router (DR) routing ST router routing ST host 224.0.0.9 RIP2 router 224.0.0.10 IGRP router 224.0.0.12 DHCP server/relay proxy 224.0.0.13 all PIM routers 224.0.0.15 all CBT routers 224.0.0.18 then to routing are available. Other suggestions are retained for devices or hosts on the network. it should also be noted that the actually reserved address space is far more than those. IANA also reserves the address range 239.0.0.0 -- 239.255.255.255 as the management range address for use in private multicast domains. multicast MAC address xxxxxx11.xxxxxxxx. xxxxxxxx. xxxxxxxx. xxxxxxxx. we all know that the xxxxxxxxMAC address is 48 bits. If the last two bits in the first eight bits are set to 1, it is set to the MAC address of multicast. ing between www.2cto.com Ethernet IP address and multicast MAC address

Because the 4-bit high of an IPv4 multicast address is 1110, it represents the multicast ID, while only 23 of the 28-bit low are mapped to the IPv4 multicast MAC address, in this way, the IPv4 multicast address will lose the information of five digits. Therefore, 32 IPv4 multicast addresses are mapped to the same IPv4 multicast MAC address. Therefore, during Layer 2 processing, the device may need to receive multicast data other than the IPv4 multicast group, and the excess multicast data needs to be filtered by the upper layer of the device. NOTES: As you can see, layer-3 multicast IP addresses start with 1110. The range is from 1110 0000-1110 1111, that is, 224 to 239. the multicast MAC starts with 0x010005E. at last, we can see that the layer-3 multicast IP address starting with 224-239 is mapped to the layer-2 multicast MAC address. here, a problem arises. Performance impact of MAC address ing: because all 28 bits of the layer-3 IP multicast address information cannot be mapped to the available mac address space of 23bit, during the ing process, 5-bit address information is lost, which causes the multicast address to be mapped to the second layer ieee mac address. The address of 2 is 5 to the power, or the address of 32: 1 is not clear. this means that each ieee ip multicast MAC address can represent 32 IP address multicast addresses. the last 23 digits of MAC and IP correspond one to one, and the last 24th digits can be 0 or 1. Each layer-2 address can be mapped to 32 layer-3 addresses. 0100.5e01.01010100.5e can be mapped to the IP address's 1st Bytes: 224-23901.0101 can be mapped to the last three bytes of the IP Address: 1.1.1 and 129.1.1. This MAC address can be mapped to: 224.1.1.1, 224.129.1.1, 225.1.1.1, 225.12 .... 239.129.1.1. Remember that the customer had such a requirement some time ago. Multicast-when issuing command "multicast mac-address 01: 00: 5e: 01: 01: 01 vlan 30 interface ethernet 0/0/1 "-we state 239.1.1.1 as MAC address-they what to state it as IP address. www.2cto.com, in fact, this requirement is that on a layer-2 switch, the customer does not want to use a 48-bit layer-2 multicast MAC address, which is too troublesome. This is understandable, many customers simply cannot remember that the 25 bits in front of the MAC address start with 0x01005E, but they cannot clearly understand the corresponding relationship. Therefore, the customer wants to write a layer-3 multicast IP address on the layer-2 switch so that the system can automatically translate the IP address into a layer-2 multicast MAC address. the mac principle of layer-2 multicast is known here. A major problem is that a MAC can correspond to 32 multicast IP addresses at the same time. the final implementation method of the software is: no matter what IP address the customer writes, such as 224/239/225/226. then the command line mapped to the system will automatically become 224. here we will give the customer a warning command line to explain the problem, and the system will finally identify the first 25 digits of 01005E. the customer's requirements have been met, but the problem must be clarified to the customer. author: hny2000