Multicasting
The destination address of IP multicast is different from that of unicast. The destination address of IP multicast is group address-class D address.
Class D addresses are IP addresses from 224.0.0.0 to 239.255.255.255
Where 224.0.0.0 to 224.0.0.255 are reserved addresses
224.0.0.1 represents all multicast groups in the subnet
224.0.0.2 represents all routers in the subnet
224.0.0.5 indicates an OSPF (Open Shortest Path First) router
224.0.0.6 indicates that the OSPF designated router
224.0.0.12 represents a DHCP (Dynamic Host Configuration Protocol) server.
Class D addresses are transient addresses that are dynamically assigned and restored. Each multicast group corresponds to a dynamically assigned class D address, and when the multicast group ends the multicast, the corresponding class D address will be recycled for future multicast. In the allocation of class D addresses, IETF recommends the following principles:
Worldwide: 224.0.1.0 to 238.255.255.255
Limited range: 239.0.0.00239.255.255.255
Local site scope: 239.253.0.0" 239.253.0.16
Scope of local institutions: 239.192.0.039.192.0.14.
How do layer 2 MAC addresses map to layer 3 IP addresses? The first 25 bits of the MAC address are forced to specify the bit 0100.5e, and the last 23 bits correspond to the last 23 bits of the IP address, while the first 4 bits of the multicast IP address are the same as:
IP address: 1110yyyy.yxxxxxxx.xxxxxxx.xxxxxxxx
MAC address: 00000001.00000000.01011110.0xxxxxxx.xxxxxxx.xxxxxxxx
For example, which of the following multicast IP addresses should be mapped to the 224.215.145.230 multicast MAC address? (.)
(a) 01-00-5e-57-91-e6 (B) 01-00-5e-d7-91-e6
(C) 01-00-5e-5b-91-e6 (D) 01-00-5e-55-91-e6
In binary system, 215.145.230 is converted into 1101Magi 0111Magi 1001je 0001Magi 1110le 0110. Taking the last 23 bits in the MAC address, we can calculate that the answer is A.
Obviously there are 32 IP addresses (5 y can be different) corresponding to one MAC address, so avoid multiple multicast IP addresses used in the same network corresponding to one MAC address
