/mcpMulticast IP addresses are Class D IP addresses. Therefore, all IP addresses from 224.0.0.0 to 239.255.255.255 are multicast IP addresses. They are also referred to as Group Destination Addresses (GDA). 1. For each GDA there is an associated MAC address. This MAC address is formed by 01-00- 5e, followed by the last 23 bits of the GDA translated into hex, as shown below.239.20.20.20 corresponds to MAC 01-00-5e-14-14-14.239.10.10.10 corresponds to MAC 01-00-5e-0a-0a- 0a.Consequently, this is not a one-to-one mapping, but a one-to-many mapping. From these two addresses, you can see that the first octet (239) is not used in the MAC address. So the multicast addresses with the same last three octet but different first octet have overlapping MAC addresses. 2. Some Multicast IP addresses are reserved for special use, as shown below.224.0.0.1 - All multicast-capable hosts.224.0.0.2 - All multicast-capable routers.224.0.0.5 and 224.0.0.6 are used by Open Shortest Path First (OSPF). 3. In general, addresses from 224.0.0.1 to 224.0.0.255 are reserved and used by various protocols (standard or proprietary, such as Hot Standby Router Protocol (HSRP)). Cisco recommends that you not use these for GDA in a multicast network. CGMP and IGMP snooping do not work with this reserved address range. Internet Group Management Protocol IGMP is a standard defined in RFC1112 for IGMPv1, in RFC2236 for IGMPv2 and in RFC3376 for IGMPv3. IGMP specifies how a host can register with a router in order to receive specific multicast traffic. The next section gives a brief overview on IGMP. IGMPv1 IGMP Version 1 (IGMPv1) messages are transmitted in IP datagrams and contain the following fields: Version: 1 G Type: There are two types of IGMP messages, Membership Query and Membership Report. G Checksum G GDA G Membership reports are issued by hosts that want to receive a specific multicast group (GDA). Membership queries are issued by routers at regular intervals to check whether there is still a host interested in the GDA in that segment. Host membership reports are issued either unsolicited (when the host wants to receive GDA traffic first) or in response to a membership query. They are sent with the following fields: L2 Information Source MAC: Host MAC address G Destination MAC: Destination MAC for the GDA G L3 Information Source IP: IP address of the host G