B&B Electronics 516TX-A - Manual - Manual (Page 48)

(Revised 8/4/2009) 48
First, the class D address must be known. While some of these class D addresses are well known (224.0.1.1
is used by NTP Network Time Protocol specified in RFC1119 for instance), the majority are defined when
the application is activated. Often, the class D address is the originating host computer’s IP address.
In most applications, the user has the option of choosing their multicast group address as a session is
established. The complete process of establishing IP multicast sessions is fully documented in RFC 1112
"Host Extensions for IP Multicasting" and the IGMP Version 2 RFC 2236. .
For the purposes of this document, only the process used to map a selected IP multicast class D address to a
specific MAC layer multicast destination address is significant.
Once an application determines the class D IP multicast address it will utilize, that address must be mapped
into a MAC layer multicast for delivery across any LAN based system. This process is outlined as follows:
Step 1: Using the Class D address, identify the low order 23 bits of the class D address.
Step2: Map those 23 bits into the low order 23 bits of a MAC address with the fixed high order 25 bits of the
IEEE multicast addressing space prefixed by 01:00:5E.
An example of this process is shown below. This example assumes that the application has selected the IP
class D address 239.1.1.10 and demonstrates the mapping of the low order 23 bits of the class D IP address
to the low order bits of the MAC layer destination.
Mapping of class D 226.129.1.10 IP address to MAC layer multicast
The mapping of 226.129.1.10 to a MAC layer multicast was executed by placing the low order 23 bits of the
class D address into the low order 23 bits of the reserved MAC layer multicast address 01:00:5E:xx:xx:xx.
Since only 23 bits are mapped, the 24th significant bit is fixed at 0. The final MAC address that is utilized by
the multicast group 226.129.1.10 is 01:00:5E:01:01:0A.