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.
Predict traffic delivery
Predict the switch egress ports, interfaces that physically receive the frame, hosts that accept it, and the router-boundary action. Use I know this—proceed to advanced when the foundational cases are already familiar.
Predict traffic delivery
Destination: Local server
Diagnose delivery-scope scenarios
Use evidence in order: classify the destination, identify ingress, check learned MAC or receiver-group state, trace eligible egress ports, then separate receipt from acceptance.
Unexpected flooding
A missing or aged MAC entry can temporarily flood a unicast frame; it does not turn the destination into broadcast.
Broadcast and relay boundaries
An ordinary Layer 2 broadcast stops at a router. For DHCP, verify whether a configured relay received the local request and generated a separate forwarded request toward the server.
Multicast symptoms
An unsubscribed host may capture multicast when the switch lacks receiver-group state, then reject it. For another network, verify that multicast routing is explicitly enabled; addressing alone does not create routed distribution.
Packet evidence
Capture at more than one observation point when possible. Record destination addressing, ingress and egress links, interface receipt, and host processing. A capture proves receipt at that point—not application acceptance.
Knowledge check and summary
| Scope | Switch behavior | Acceptance |
|---|---|---|
| Known unicast | Learned port | One destination |
| Unknown unicast | Eligible non-ingress ports | One destination |
| Broadcast | Local eligible ports | Local participants inspect it |
| Multicast | Subscriber ports when state is known; otherwise local flooding may occur | Joined receivers |
Interview scenario: explain why seeing a frame in a capture does not prove an application accepted it. Separate switch forwarding, interface receipt, operating-system filtering, and application processing.