Multicast Addressing

Because networking devices talk to each other with both Layer 3 and Layer 2 addresses, multicasting must address both these issues. There must be a Layer 3 destination address that the multicast server can use to send out its traffic feed to any participating end stations. There also must be a Layer 2 address that NICs can use to correctly process the information. Within IP, Class D addresses have been reserved for Layer 3 addressing.

With Class D addresses, the first four high-order bits are 1110, providing addresses ranging from 224.0.0.0 through 239.255.255.255. To provide a Layer 2 MAC address, part of the corresponding IP multicast address will be mapped to the end of a reserved range of MAC addresses: 28 bits.

The Internet Assigned Numbers Authority (IANA) is responsible for the assignment of multicast addresses for vendors that need them for multicast applications. Multicast addresses are broken into the following groups:

• Reserved link local addresses They range from 224.0.0.0 224.0.0.255, which are used to communicate with devices on the same LAN segment. The TTL (Time-To-Live) field is set to 1 and routers never forward these multicasts. Table 8.1 displays some of the most often used reserved link local multicast addresses.

• Globally scoped addresses They range from 224.0.1.0 238.255.255.255, and are like public IP addresses: They're used to transmit multicast information across a public network.

• Source-specific addresses They range from 232.0.0.0 232.255.255.255. They're used by an extension of the Protocol Independent Multicast (PIM) routing protocol, called Source Specific Multicast (SSM).

• GLOP addresses They range from 233.0.0.0 233.255.255.255, reserved in RFC 2770. These addresses are reserved for sites with assigned autonomous system (AS) numbers. The autonomous system number (16 bits) is converted into hexadecimal and the resulting four hexadecimal digits are broken into two sets. These two hexadecimal sets of numbers are then converted to decimal and are inserted into the second and third octets of the multicast address. The fourth octet can then be used for specific multicast addresses for the AS. For example, if you had an AS number of 62009, it would be F230 in hex. Convert F2 and 30 to decimal and you have 242 and 57, which results in the following multicast range: 233.242.57.0 233.242.57 255.

• Limited score addresses They range from 239.0.0.0 239.255.255.255. They're private multicast addresses assigned in RFC 2365 and can be used only within an organization: They cannot be used in a public network. In this sense, they're like private IP addresses.

Table 8.1. Common Multicast Addresses

Address	Description
224.0.0.1	Used to send information to all multicast devices in a subnet
224.0.0.2	Used to send information to all multicast RPs in a subnet
224.0.0.5	Used by OSPF routers
224.0.0.6	Used to send routing information to OSPF designated routers
224.0.0.13	Used by PIM RPs

All messages sent to the group address 224.0.0.1 (the all-host group) have their TTL field in the IP packet set to 1. This ensures that the RP will not forward them to other segments.