IPv6 Network Administration

Broadcast Versus Multicast

Another feature of ARP is that it is a broadcast protocol, which means its transactions are heard network-wide. Every time a host receives a broadcast, it must process the packet, even if, as must be the case in sufficiently large networks, the packet has nothing to do with the host. This can add up to phenomenal amounts of traffic on (badly-designed) flat networks. (There are urban legends floating around of a network on which it is impossible to plug in machines of below a certain specification, because they do not have the processing power to deal with the volume of ARP and other broadcast packets. If you find such a network, let us know.)

Multicast is an entirely more sensible way to hold multiway conversations. Multicast allows you to address a group of hosts interested in a particular type of network traffic without disturbing uninterested bystanders. The Section 3.3.4 talks more about IPv6 multicast.

Unfortunately multicast never really took off on IPv4 networks. There are a variety of likely reasons for this: it's not enabled by default, it takes significant work to configure and most of the target applications for multicast involve cooperation across multiple administrations. This is a pity, as multicast is useful and well-designed.