Lesson
Unicast, Broadcast and Multicast Communication
Learning objective
Identify a transmission's delivery scope and predict which interfaces receive, accept, or forward it.
Why delivery scope matters
Delivery scope answers a practical question: which interfaces receive a transmission, and which hosts actually accept and process it? These are separate events. A switch forwards through ports, interfaces receive bits, and a host decides whether the destination is relevant.
Unicast: one intended destination
Unicast identifies one intended destination, such as your laptop sending an HTTPS request to a web server. When the destination MAC address is already learned, a switch selects one egress port. A switch may forward an unknown unicast through several ports while it learns, but the destination remains unicast.
Broadcast: the local broadcast domain
A broadcast is delivered throughout its local broadcast domain. An ARP request is a familiar example: every local interface receives it, but only the device owning the requested address replies. A router can receive the frame on a local interface, but does not ordinarily forward it into another network. A DHCP relay is a configured agent that creates or forwards a new request toward a server; it is not an ordinary broadcast crossing the router unchanged.
Multicast: an interested receiver group
Multicast addresses a receiver group, such as viewers subscribed to the same live video stream. Hosts accept the traffic only when they have joined that group, while efficient forwarding depends on group-aware network state. Without that state, local flooding can make a non-subscriber interface receive a frame that its host then rejects.
Unknown unicast is not broadcast
Flooding describes a switch action. Unicast, broadcast, and multicast describe destination scope. Flooding does not rewrite the original unicast destination into a broadcast address.
Compare delivery types
Compare who the sender intends to reach, what the switch forwards, what each interface physically receives, and what each host accepts.
| Delivery type | Intended destination | Typical local forwarding | Host acceptance |
|---|---|---|---|
| Known unicast | One host | One learned port | The matching destination |
| Unknown unicast | One host | Eligible ports except ingress | Still only the matching destination |
| Broadcast | Local broadcast domain | Eligible ports except ingress | Local participants inspect it |
| Multicast | Interested receiver group | Subscriber ports when state is known; otherwise it may flood locally | Joined receivers |
Interactive delivery-scope player
Trace who receives—and who accepts—the traffic
Destination scope: Unicast destination: Host B
Switch cue: known-unicast
Forwarded
P2
Received
Host B — subscriber
Accepted
Host B — subscriber
Router boundary
not in path
The learned unicast destination uses only P2.
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