Unicast, Multicast, Broadcast

Unicast
A frame with a destination unicast MAC address is intended for just one device on a segment. The top part of Figure 2-2 shows an example of a unicast. In this example, PC-A creates an Ethernet frame with a destination MAC address that contains PC-C’s address. When PC-A places this data link layer frame on the wire, all the devices on the segment receive. Each of the NICs of PC-B, PC-C, and PC-D examine the destination MAC address in the frame. In this instance, only PC-C’s NIC will process the frame, since the destination MAC address in the frame matches the MAC address of its NIC. PC-B and PC-D will ignore the frame.

Multicast

Unlike a unicast address, a multicast address represents a group of devices on a segment. The multicast group can contain anywhere from no devices to every device on a segment. One of the interesting things about multicasting is that the membership of a group is dynamic—devices can join and leave as they please. The detailed process of multicasting is beyond the scope of this book, however.

The middle portion of Figure 2-2 shows an example of a multicast. In this example, PC-A sends a data link layer frame to a multicast group on its segment. Currently, only PC-A, PC-C, and PC-D are members of this group. When each of the PCs receives the frame, its NIC examines the destination MAC address in the data link layer frame. In this example, PC-B ignores the frame, since it is not a member of the group. However, PC-C and PC-D will process the frame.

Broadcast

A broadcast is a data link layer frame that is intended for every networking device on the same segment. The bottom portion of Figure 2-2 shows an example of a broadcast. In this example, PC-A puts a broadcast address in the destination field of the data link layer frame. For MAC broadcasts, all of the bit positions in the address are enabled, making the address FFFF.FFFF.FFFF in hexadecimal. This frame is then placed on the wire. Notice that in this example, when PC-B, PC-C, and PC-D receive the frame, they all process it.

Broadcasts are mainly used in two situations. First, broadcasts are more effective than unicasts if you need to send the same information to every machine. With a unicast, you would have to create a separate frame for each machine on the segment; with a broadcast, you could accomplish the same thing with one frame. Second, broadcasts are used to discover the unicast address of a device. For instance, when you turn on your PC, initially, it doesn’t know about any MAC addresses of any other machines on the network. A broadcast can be used to discover the MAC addresses of these machines, since they will all process the broadcast frame. In IP, the Address Resolution Protocol (ARP) uses this process to discover another device’s MAC address.