Internet Core Protocols: The Definitive Guide

Local Multicasting

Remember that IP datagrams (and IP addresses) are used only to represent a consistent view between IP-enabled devices, and that when two devices on the same local network want to communicate with each other they will use the topology-specific protocols and addresses defined for the local network medium to actually exchange data. For example, two hosts on the same local Ethernet segment will use Ethernet framing and addressing mechanisms when they want to exchange IP data, with the IP packets being passed in the Ethernet frames. Meanwhile, hosts on Token Ring networks will use the protocols and addressing mechanisms specified in the Token Ring standards when they want to exchange data.

These same rules apply with multicast traffic, which must also use the low-level protocols and addressing rules defined for whatever medium is in use with the local network. When multicast IP packets are being passed around the network, they are really being sent in a form that uses the multicasting services that are provided by the underlying network topology.

Most shared-medium networks provide three distinct types of addressing:

Unicast

Unicast data is sent from one device to another, using an explicit destination address. Devices on the network must monitor the network traffic, looking for any frames that are marked for their local hardware address. With Ethernet, this is a 48-bit address typically written in hexadecimal notation as something like c0:14:3d:22:1e:04.

Broadcast

Broadcast ...