Network Basics: Network Destination Addresses
The data link layer provides the means to transfer data between all network devices, including Cisco components, using network destination addresses. These data link layer addresses have their own set of address classifications: unicast, multicast, and broadcast. These three classifications of addresses also apply to the network layer, and specifically to the IP protocol. At the network level, here is how these three address classes work:
Unicast: Unique IP address from the Class A, B, or C address ranges. A unicast IP address is always associated with a MAC address for the network interface on which it operates. So if your network device has a network interface for which you have configured an IP address, the IP address uniquely identifies your computer at the network layer.
At the same time, at the data link layer, the MAC address associated with the configured network interface uniquely identifies the network device at the data link layer. So there is a one-to-one relationship between unicast addresses at the data link and network layers, and data being sent to a unicast address is processed only by one device on the network.
Multicast: Of the three IP address categories, this is the one that is tough to understand. A multicast address, which is a Class D address, is associated with a group of network devices. When you send data to the multicast address, it is received by all the devices that are a member of the multicast group. Unlike the unicast address, multiple network devices can be configured to receive data sent to this address.
This is useful when sending the same data to multiple devices, such as streaming music or installing a disk image on 50 computers. Instead of sending 50 copies of the data to 50 unicast addresses, multicast allows you to send one copy of the data, and all 50 computers receive it. Unlike broadcast, covered in just a bit, this data is processed only by the devices that are in the group, and not all devices on the network.
At the data link layer, IANA has registered an Organizationally Unique Identifier (OUI) like how a network card manufacturer would register. In this case, instead of using this MAC address block to be assigned to network cards, IANA uses this MAC address range (01:00:5e:00:00:00 to 01:00:5e:7f:ff:ff) to associate IP multicast addresses at the network layer to MAC addresses at the data link layer.
This MAC address range is a little smaller than the IP address range, so there is small potential overlap, but this does not present an issue for the multicast traffic.
The registration of the IANA MAC range allows an IP multicast address to map to a MAC address at the data link layer, but it does require that your network switch supports the IANA multicast range; if it does not, the switch treats IP multicast traffic as broadcast traffic (see the next point) at the data link layer.
Broadcast: Special group IP addresses technically include all IP network devices in the world but actually map directly to the MAC broadcast address on your current data link.
If you look at the broadcast IP address in binary, all the bits are set to 1, which is 255.255.255.255 in the standard dotted decimal notation. The broadcast MAC address that this IP broadcast maps to also has all its binary bits set to 1, so the address is FF:FF:FF:FF:FF:FF.
Any network frames that are sent to the broadcast address are read or processed by every device on that network segment.