Traffic on a network fundamentally flows in one of three ways depending on the destination intent.
1. Unicast
Unicast is one-to-one communication. A single sender transmits data to a single specific destination.
- Example: Your PC loading a web page from a specific Web Server. Most internet traffic is unicast.
2. Broadcast
Broadcast is one-to-all communication. A single sender transmits data to every device within the local broadcast domain (typically synonymous with a VLAN).
- Example: An ARP request where a PC asks the entire network, "Who has IP 192.168.1.1?"
- Important Distinction: IPv6 does NOT use broadcast traffic. IPv6 relies entirely on targeted multicast to achieve similar goals.
3. Multicast
Multicast is one-to-many communication. A single sender transmits one copy of the data, and the network hardware replicates it only to the specific group of devices that have explicitly requested to receive it.
- Example: A live video stream being sent to hundreds of employees. Sending unicast would overwhelm the server; sending broadcast would overwhelm the entire network. Multicast efficiently sends one stream that is duplicated at the edge switches.
- Management: IPv4 relies conceptually on IGMP (Internet Group Management Protocol) for clients to join a multicast group. IPv6 relies on MLD (Multicast Listener Discovery).