Configuring IP Multicast
IP multicast address structure

Multicast uses Class D IP address space. A Class D address consists of 1110 as the high-order bits in the first octet, followed by a 28-bit group address. The range of IP multicast addresses is divided into classes based on the high-order bits of a 32-bit IP address.

The remaining 28 bits of the IP address identify the multicast group ID. This multicast group ID is a single address typically written as decimal numbers in the range 224.0.0.0 through 239.255.255.255. The high-order bits in the first octet identify this 224-base address.

Multicast addresses may be dynamically or statically allocated. Dynamic multicast addressing provides applications with a group address on demand. Dynamic multicast addresses have a specific lifetime and applications must request and use the address as needed. Static addresses are used at all times. As with IP addressing, there is the concept of private address space that may be used for local, organization wide traffic and public or Internet wide multicast addresses. There are also addresses reserved for specific protocols that require well-known addresses. The Internet Assigned Numbers Authority (IANA) manages the assignment of multicast addresses that are called permanent host groups and are similar in concept to well known TCP and UDP port numbers.

IP Multicast to MAC Address Mapping
Due to decisions taken early in the development of multicasting, only the MAC address range from 0100.5e00.0000 through 0100.5e7f.ffff is the available for carrying multicast frames.

This makes the first 25 bits of the MAC address fixed and allows for the last 23 bits of the MAC address to correspond to the last 23 bits in the IP multicast group address.

Because the upper five bits of the IP multicast address are dropped in this mapping, the result is that two different IP Multicast addresses may map to the same MAC multicast address. For example, 224.1.1.1 and 225.1.1.1 map to the same multicast MAC address. If one user subscribed to Group A (as designated by 224.1.1.1) and the other users subscribed to Group B (as designated by 225.1.1.1), they would both receive both A and B streams at Layer 2. At Layer 3, however, only the packets associated with the IP address of the selected multicast group would be viewable because the port ranges used within the address will be different between aliased streams. Network administrators should consider this when assigning IP multicast addresses.