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224.0.0.17 All SBMS
224.0.0.18 VRRP
When transferring unicast IP packets on Ethernet, the destination MAC address is the
MAC of the receiver. However, in transferring multicast packets, as the destination is no
longer one specific recipient but a group with unknown members, the destination address
used Is the multicast MAC address. Multicast MAC address is corresponding 5to the
multicast IP address. According to IANA (Internet Assigned Number Authority), the 24
MSbs of multicast MAC is 0x01005e and 23 LSbs of multicast MAC is the same of the
multicast IP address.
As only 23 bits out of the 28 LSbs of multicast IP address are mapped to MAC address, for
one MAC address there will be 32 corresponding multicast IP addresses.
In the multicast model, the source host sends information to the host group represented
by the multicast group address in the destination address field of the IP packet. The
multicast model differs from the unicast model in that a multicast packet must be
forwarded to several external interfaces to send the packet to all receiving stations, i.e.
multicast forwarding is more complex than unicast forwarding.
To ensure the multicast packets reach the routers in the shortest route, the multicast
protocols must check the receiving interfaces of the multicast packets against the unicast
route table or route table dedicated for multicast (such as a DVMRP route table). Such
check mechanism is the base for most multicast routing protocols to perform forwarding,
and is called Reverse Path Forwarding (RPF) check. Multicast routers use the source
address of an arrived multicast packet to query the unicast route table or an independent
multicast route table to make sure the ingress interface at which packet arrived is in the
shortest route from the receiving station to the source address. If an active tree is used,
the source address is the address of source host sending the multicast packet; if a shared
tree is used, the source address is the root address of that shared tree. When a multicast
packet arrives at a router, the packet will be forwarded according to the multicast
forwarding rules if the RPF check ok; otherwise, the packet will be discarded.
IP multicast technology effectively solved the problem of one sender vs. multiple receivers,
fulfilling the high efficiency data transmission from one point to multiple points in the IP