When you assign a Layer 3 multicast address, a
Layer 2 address is automatically generated from the IP address. The
Figure
shows how a MAC multicast address is derived from an IP multicast
address. To calculate the Layer 2 address, the host copies the last
23 bits of the IP address into the last 24 bits of the MAC address.
The high-order bit (of these 24 bits) is set to 0.
The first 3 bytes (24 bits) of the
multicast MAC address are 0x01-00-5E. This reserved value indicates a multicast application.
Consider an example: The IP address
224.1.10.10, assigned by an administrator, has a low 23-bit value of
1.10.10. In hexadecimal format, this is 0x01-0A-0A. The MAC address
takes the last 23 of the 24 bits and places them into the MAC field.
The complete MAC address in this case is 01-00-5E-01-0A-0A.
Figure
illustrates another example. In this case, the IP address 224.10.8.5
translates to a MAC address of 01.00.5e.0a.08.05.