Study Guide
Unicast, Multicast, Broadcast and Anycast
The four ways traffic can be addressed, what a broadcast domain is, and why routers stop ordinary broadcasts.
Lesson 3.3 Networking Fundamentals
When a device sends something, it has to say who it is for. This lesson is about the choices it has, and why the choice matters for performance and for security.
Stage: Think
Who is it for?
Think it through
A teacher in a busy room has three things to say: a private word to one pupil, instructions for one table working on a project, and “Pens down, everyone.” How does she reach each audience?
- Hint 1: Who needs to hear each message, and who does not?
Show what happens
Quietly to the one pupil, to that table only, and loudly to the whole room. A device sending data has the same three choices.
Think it through
Show the answer
Answer: B. Ask everyone nearby; the printer answers.
Ask everyone nearby and let the printer answer. Asking everyone is one of the delivery methods, and the next step names it.
Stage: Explain
Who receives the traffic?
Picture a teacher in a room full of pupils. She can speak quietly to one pupil, speak to one table of pupils working on the same task, or address the whole room. Networks have the same three choices.
Unicast: one to one. The message is addressed to a single device. Almost everything you do is unicast: when your browser asks for a web page, the request goes to one web server and the page comes back to your one device.
Multicast: one to a chosen group. The message goes to every device that has asked to be in a particular group, and to no others. A live lesson streamed to the thirty computers that joined it is multicast: one sender, thirty receivers, and the rest of the network is left alone.
Broadcast: one to everyone nearby. The message goes to every device on the local network, whether or not it wants it. A device does this when it does not yet know whom to ask. A laptop that needs the printer, and knows the printer’s name and not where it is, effectively calls out “which of you is the printer?” to the whole network.
The group that a broadcast reaches is called a broadcast domain: all the devices that hear one another’s broadcasts. It is the “room” in the comparison. A router marks its edge, because routers do not normally pass broadcasts on to other networks.
Where the comparison stops. In a room, everyone hears everything and politely ignores what is not for them. On a modern network, a unicast message is delivered only to its recipient: other devices do not normally receive it at all.
Two facts to carry forward. The newer version of the internet’s addressing system, IPv6, has no broadcast and uses multicast for the same jobs. And how a message is delivered is a separate question from which layer its protocol belongs to: a protocol can sit high in the layer model and still use broadcast.
The fictional school network from What Is a Network?: every device is joined straight to the main switch, except the laptop, which joins the wireless access point by Wi-Fi, and the internet, which joins the router.
- Solid line: cable
- Dashed line: Wi-Fi
- Double line: the internet line
- Internet is joined to Router by the internet line.
- Router is joined to Main switch by cable.
- Office computer is joined to Main switch by cable.
- Printer is joined to Main switch by cable.
- School server is joined to Main switch by cable.
- Access point is joined to Main switch by cable.
- Teacher’s laptop is joined to Access point by Wi-Fi.
A simplified picture. Real networks have more switches, and a router can sit inside a network as well as at its edge.
Three deliveries on the school network
- Unicast: one device to one device. The office computer sends a print job to the printer. The switch sends it down the printer’s cable and nowhere else, so only the printer receives it.
- Broadcast: one device to everyone in the broadcast domain. The laptop asks “which of you is the printer?”. The switch sends the question to every device connected to it: the office computer, the printer, the server and the router. The router does not pass it on to the internet: it marks the edge of the broadcast domain.
- Multicast: one device to a chosen group. The school server streams a lesson to the devices that have joined it: here the office computer and the teacher’s laptop. The printer has not joined, so it does not receive the stream. (A switch that keeps track of which devices have joined does this; a simpler switch may send multicast to every device, as it does a broadcast.)
Check your understanding
Show the answer
Answer: C. Broadcast: everyone nearby gets it.
Broadcast: it does not yet know which device can help, so it asks every device in its broadcast domain.
Stage: Predict
Why broadcasts stay local
Think it through
Show the answer
Answer: B. Every device would be swamped.
Every device would be overwhelmed. That is why routers do not normally pass broadcasts on.
Every device in a broadcast domain has to receive and look at every broadcast, even when it is not for them. On a small network that costs little. On a large one, broadcasts would soon swamp every device if they travelled everywhere. That is why broadcasts are limited: routers do not normally pass them between networks, and one physical network can be divided into several smaller broadcast domains called VLANs, which a later lesson explains.
This also has a security side. Anything sent by broadcast is seen by every device in the domain, including any device an attacker controls. Two protocols taught in DHCP and ARP use broadcasts when a device first joins a network (one to get an address, one to find other devices), which is part of why they can be abused locally.
Stage: Explain
Anycast
Anycast completes the picture. Several servers in different places share the same address, and routing delivers each request to the nearest or best-placed one. To the sender it looks like unicast: one request goes to one server. Some large DNS and content-delivery services use anycast so that users reach a nearby server.
For each case, ask one question: who is meant to receive it?
- A laptop joins a network and asks “Who has 192.168.1.20?” It does not yet know which device owns that address, so it asks everyone on the local network: broadcast. (This particular question is an ARP request, taught in a later lesson.)
- The printer answers the laptop directly. The printer now knows exactly who asked, so the reply is for one device: unicast.
- A lecture is streamed to the thirty computers that have joined the stream. One sender, a chosen group of receivers: multicast.
- A user’s DNS query is answered by the closest of many servers sharing one address. One request reaches one of several servers chosen by routing: anycast.
What the four cases show: the method follows from what the sender knows. Not knowing who to ask forces a broadcast; knowing exactly who allows a unicast.
When a network is slow or noisy, the volume of broadcast traffic is worth checking. A misbehaving device, or a very large broadcast domain, can flood every device with traffic it has to process.
Stage: Apply
Apply it in a new situation
These questions use the lesson in situations it has not shown. For the strongest test of your understanding, come back to them on another day and answer each one at the first attempt.
Apply it
- Hint 1: Which kind of traffic must every device in the domain look at?
Show the answer
Answer: A. Broadcast overload; split it into smaller domains.
Broadcast traffic grows with the size of the domain, and every device must process it. Splitting the network into smaller broadcast domains reduces it.
Stage: Check
Check your understanding
These questions come from the approved revision bank. Choose an answer and check it; the explanation and its sources follow.
Show answer
Answer: Multicast
Multicast targets group members. Broadcast targets the broadcast domain; unicast targets one recipient. NAT is address translation, not a traffic delivery category.
Revision sources: Session 5: Networking Fundamentals — Part 1 (Classroom session record)
Show answer
Answer: A phone and Bluetooth headphones can form a PAN.; A LAN can use Wi-Fi as well as wired Ethernet.
PAN describes personal scope, and LANs can be wired or wireless. Ordinary local broadcasts normally remain within their broadcast domain. Scope labels do not specify universal distance limits.
Revision sources: Session 5: Networking Fundamentals — Part 1 (Classroom session record)
Study and revision
Practise delivery methods through the Networking & Protocols topic.
Stage: Consolidate
What to remember
- Unicast is one to one, multicast one to a chosen group, broadcast one to everyone nearby, and anycast one to the nearest of several.
- A broadcast domain is all the devices that hear each other’s broadcasts; a router marks its edge.
- Broadcasts stay local because every device must process them, and every device in the domain can see them.
- The method follows from what the sender knows: not knowing whom to ask forces a broadcast.
- IPv6 has no broadcast, and delivery method is a separate question from OSI layer.
Key terms
- Unicast
Communication sent to one intended recipient.
- Multicast
Communication sent to a selected group of recipients.
- Broadcast
Communication sent to every device in the relevant broadcast domain. Routers do not normally forward ordinary local broadcasts.
- Broadcast domain
The set of devices that receive one another’s local broadcasts. It is often bounded by a VLAN, and routers do not normally forward ordinary broadcasts between domains.
Common misunderstandings
Ordinary broadcasts stay within their broadcast domain, which is usually one network or VLAN. Routers do not normally forward them.
Delivery method and OSI layer are separate questions. DHCP uses broadcast but is an application-layer protocol.
IPv6 has no broadcast. It uses multicast for tasks that IPv4 handles with broadcast.
Lesson sources and acknowledgements
Sources and scope
- Taught in Session 5 (see the Session 5 Learning Log): unicast, multicast and broadcast, broadcast domains, and routers not forwarding ordinary broadcasts.
- Moved from an earlier lesson: “Who receives the traffic?” was previously part of What Is a Network? and keeps its heading and anchor here; its wording was rewritten on 13 October 2026.
- Supplementary explanation: the teacher-and-pupils comparison, anycast, why broadcasts are kept local, the identification example and the misunderstandings.
Guided version (published 1 November 2026, awaiting the site owner’s editorial review): the room and printer opening questions, every question asked in the lesson with its feedback and hints, the three deliveries traced on the school network, the address request, the 2,000-device network and the DHCP claim, and the closing summary are supplementary explanation written for the Study Guide. The settings are fictional. The questions in the lesson’s steps are not revision-bank items; those in “Check your understanding” are, unchanged.
Delivery to the nearest, or best-placed, of several devices that share the same address, as decided by routing.
A protocol that finds the MAC address for an IPv4 address on the local network, using a broadcast request and a normally unicast reply.
Communication sent to every device in the relevant broadcast domain. Routers do not normally forward ordinary local broadcasts.
The set of devices that receive one another’s local broadcasts. It is often bounded by a VLAN, and routers do not normally forward ordinary broadcasts between domains.
A measure intended to modify risk, for example by restricting access, detecting misuse or supporting recovery.
An application-layer protocol that automatically provides IP configuration, such as an address, subnet mask, default gateway and DNS server information.
The system that translates domain names into IP addresses. It commonly uses port 53 over UDP and TCP.
Communication sent to a selected group of recipients.
A conceptual seven-layer reference model for describing networking functions: Application, Presentation, Session, Transport, Network, Data Link and Physical.
An agreed set of rules for how data is formatted, transmitted and interpreted so that devices can communicate.
A device that forwards traffic between networks, for example between a private local network and the internet.
Communication sent to one intended recipient.
A separate network created inside a switch, or a group of linked switches, by assigning each port to a numbered group. Devices in the same VLAN reach each other directly; devices in different VLANs can communicate only through a router.
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