When a packet is routed from one sender to one receiver, it uses unicast addressing. This is the simplest and most common type of addressing. All of the protocols we deal with in this book use unicast addressing.
Broadcast addressing allows a single sender to address a packet to all recipients simultaneously. It is typically used to deliver a packet to every receiver on an entire subnet.
If a broadcast is a one-to-all communication, then multicast is a one-to-many communication. Multicast involves some group management, and a message is addressed and delivered to members of a group.
Anycast addressed packets are used to deliver a message to one recipient when you don't care who that recipient is. This is useful if you have several servers that provide the same functionality, and you simply want one of them (you don't care which) to handle your request.
IPv4 and lower network levels support local broadcast addressing. IPv4 provides some optional (but commonly implemented) support for multicasting. IPv6 mandates multicasting support while providing additional features over IPv4's multicasting. Though IPv6 is not considered to broadcast, its multicasting functionality can essentially emulate it.
It's worth noting that these alternative addressing methods don't generally work over the broader internet. Imagine if one peer was able to broadcast a packet to every connected internet device. It would be a mess!
If you can use IP multicasting on your local network, though, it is worthwhile to implement it. Sending one IP level multicast conserves bandwidth compared to sending the same unicast message multiple times.
However, multicasting is often done at the application level. That is, when the application wants to deliver the same message to several recipients, it sends the message multiple times – once to each recipient. In Chapter 3, An In-Depth Overview of TCP Connections, we build a chat room. This chat room could be said to use application-level multicasting, but it does not take advantage of IP multicasting.
We've covered how messages are routed through a network. Now, let's see how a message knows which application is responsible for it once it arrives at a specific system.