Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 44
Class D Address

For those who answered 12.5 percent, you are correct. This is the odd thing. There are millions of Class C addresses (networks), but they only represent 12.5 percent of the available address space. Again, get those calculators out.

Class D addresses are special addresses and are known as multicast addresses. This address type is assigned to a group of network work–stations and is not assigned to represent a unique address. They are used to send IP datagrams to a group, but not all of the hosts on a network. Multicasting has many uses, including being used for addressing router update messages as well as delivering data, video, and voice over IP. Using a multicast address is a more efficient way of “broadcasting” rather than using a broadcast address, for the upper–layer software will not always be interrupted every time a broadcast packet arrives. Multicasting is different than broadcasting. With broadcasting, every station that receives the broadcast packet will automatically pass it to the upper–layer software without regard to the address. Every station that receives a broadcast packet must process it.

With a multicast address, each individual IP station must be willing to accept the multicast IP address before the transport–layer software will be interrupted. Each NIC will register a MAC layer multicast address on its adapter card, just like a unicast address (the IP address to Ethernet mapping of a multicast address is shown in a moment). In this way, the NIC can discard a packet without interrupting the upper–layer software (in most cases, anyway, some duplication of multicast addresses exist, and this too is shown in a moment). The NIC is already set up to receive a broadcast packet. This is one address known as FF–FF–FF–FF–FF–FF.

As of this writing, RFC 1700 (assigned numbers) fully explains the mapping of Class D addresses to MAC addresses and it also indicates assigned multicast addresses and registered multicast addresses.

Multicasting is completely covered in another section of this book.

Class D Address