Most internet traffic is unicast — one sender, one receiver. But there are use cases where one source needs to reach many receivers: video streams to multiple subscribers, network announcements to all hosts, service discovery. The two patterns are multicast and broadcast, and they have important differences.

This post explains both, when each is used, why IPv6 dropped broadcast entirely, and where you encounter them in 2026.

Definitions

Broadcast

Send to everyone on the local network. The packet reaches every host on the same broadcast domain (typically a single IP subnet or VLAN).

IPv4 broadcast address: 255.255.255.255 (all hosts) or the subnet’s directed broadcast (e.g., 192.168.1.255 for 192.168.1.0/24).

IPv6 has no broadcast.

Multicast

Send to a specific group of subscribed hosts. Hosts that joined the group receive the packet; others don’t.

Multicast address ranges:

IPv4: 224.0.0.0/4 (224.0.0.0 through 239.255.255.255).
IPv6: ff00::/8.

For IPv6 details, see IPv6 address types.

Comparison

Broadcast reaches all hosts on the local network whether they want it or not.
Multicast reaches only hosts that explicitly subscribed.
Unicast reaches a specific host. (See anycast vs unicast.)

Why Broadcast Is Wasteful

Broadcast was the original mechanism for “tell everyone.” Examples:

DHCP DISCOVER (a new client looking for a DHCP server).
ARP request (looking up a MAC for an IP).
Older service discovery (NetBIOS, Bonjour-era predecessors).

The problem: every host on the network processes every broadcast packet, even hosts that don’t care. On busy networks, broadcast traffic creates real overhead. Switches forward broadcasts to every port; hosts have to inspect every packet to decide if it matters.

For local-network protocols where every host needs the info (DHCP), broadcast is appropriate. For one-to-some communication, broadcast is overkill.

Multicast: Just the Subscribers

Multicast solves this by requiring subscription. A host that wants to receive a specific multicast group sends an IGMP (IPv4) or MLD (IPv6) join message. The network forwards multicast packets only to subscribers.

Example: a video stream sent to multicast group 239.1.1.1. Hosts that want the stream join the group; the multicast routing infrastructure delivers packets only to them. A host that doesn’t care about that stream never sees the packets.

Scales much better than broadcast for one-to-many.

Why IPv6 Drops Broadcast

IPv6 abandoned broadcast entirely. Anything that needed broadcast in IPv4 uses multicast in IPv6, often to “well-known” multicast groups.

Examples:

ff02::1 — All nodes on the local link. Used like “all hosts” broadcast.
ff02::2 — All routers on the local link.
ff02::1:ff??:???? — Solicited-node multicast (for neighbor discovery, replacing IPv4 ARP).

The benefit: hosts only process the multicast groups they’ve joined. The “all nodes” group is functionally a broadcast, but the design encourages selective subscription where possible.

Where Multicast Is Used

In 2026, multicast is widely used in specific niches:

IPTV

TV-over-IP networks (operator-deployed IPTV) use multicast extensively. One source streams to thousands of set-top boxes via IGMP-managed multicast trees.

Service discovery

Bonjour / mDNS, used by macOS / iOS for printer/AirPlay discovery, uses link-local multicast on 224.0.0.251 (IPv4) or ff02::fb (IPv6).

Routing protocols

OSPF, EIGRP, RIPv2 use multicast for router-to-router messages (e.g., OSPF uses 224.0.0.5).

Financial data feeds

High-volume market data (stock prices, etc.) often uses multicast within financial networks for low-latency delivery to many subscribers.

Audio/video conferencing

Inside corporate networks. Many-to-many media distribution.

Inside data centers

Some applications use multicast for fast cluster communication (Apache Cassandra used to, though most have migrated to unicast).

Multicast on the Public Internet

Notably absent from the list: the public internet. ISPs generally don’t carry multicast traffic between each other. The reasons:

Scalability concerns — Multicast routing state is complex; carriers don’t want to manage it for many tenants.
Provisioning complexity — Inter-provider multicast peering is rare.
Application-layer alternatives — Streaming services use CDNs and unicast at scale, plus anycast for distribution.

Multicast works great within a single network (corporate, ISP-internal, data center) but doesn’t span the internet. If you need to “broadcast” data to many internet users, you use a CDN with unicast streams.

Multicast Addressing

IPv4 multicast addresses have a structure:

224.0.0.0/24 — Link-local multicast (router protocols, mDNS).
224.0.1.0 – 238.255.255.255 — Globally scoped multicast (rarely used on public internet).
239.0.0.0/8 — Administratively scoped (private use within an organization).

For most internal multicast, use the 239.x.x.x range.

IPv6 multicast has scope bits and flags. The ff02::/16 scope is link-local; ff05::/16 is site-local; ff0e::/16 is global.

IGMP and MLD

IGMP (Internet Group Management Protocol, IPv4) and MLD (Multicast Listener Discovery, IPv6) are the protocols hosts use to join and leave multicast groups.

Host joins → sends IGMP Membership Report.
Router asks “anyone still want this group?” → IGMP Query.
Host stays → continues to receive.
Host leaves → IGMP Leave Group (or just stops responding to queries).

Switches with IGMP snooping inspect IGMP traffic to track which ports want which groups, then forward multicast efficiently. Without snooping, all multicast becomes effectively broadcast.

PIM and Multicast Routing

For multicast to cross routers (within a single administrative domain), you need a routing protocol:

PIM-SM (Sparse Mode) — Most common. Uses a rendezvous point.
PIM-DM (Dense Mode) — Floods then prunes. Used in dense receiver topologies.
MOSPF, DVMRP — Older; less used.

For most application developers, the routing layer is transparent. Network engineers configure PIM on routers; applications just send to the multicast address.

Application Layer Multicast

When network-layer multicast isn’t available (public internet), applications sometimes implement application-layer multicast. The pattern:

Server has many subscribers.
Server sends one copy of data to many subscribers via unicast.
Or: server sends to a few “relay” subscribers that re-distribute.

This is what video streaming services, gaming platforms, and chat services do. The network sees unicast; the application logic implements the multicast pattern.

Practical Use Cases for Developers

For most application developers in 2026, multicast and broadcast come up in:

Local network discovery

Bonjour / mDNS / DNS-SD. Your app finds local services (printers, Chromecasts, AirPlay devices) via multicast queries.

Container orchestration

Some service discovery in Kubernetes / Consul uses multicast inside clusters; most have migrated to unicast for scalability.

High-throughput internal pub/sub

Financial systems, large multi-server data feeds.

IoT

Some IoT protocols (CoAP) support multicast for discovering devices.

You almost never use raw multicast/broadcast in a typical web application.

Security Considerations

A few security points:

Broadcast amplification — Broadcast pings could be used to amplify DoS attacks (“smurf attack”). Modern OSes drop broadcast ping responses by default.
Multicast snooping — Anyone on the local network can join any link-local multicast group and snoop. Use authenticated multicast (with payload encryption) for sensitive data.
Cross-VLAN leakage — Misconfigured switches can leak multicast across VLANs.

TL;DR

Broadcast = send to everyone on the local network.
Multicast = send to a group that explicitly subscribed.
IPv6 has no broadcast — all “one-to-many” is multicast.
Multicast scales better than broadcast for one-to-many.
Public internet doesn’t carry multicast between ISPs.
Used heavily in IPTV, service discovery (mDNS/Bonjour), routing protocols.
IGMP / MLD manage group membership.
For typical web apps, multicast isn’t a concern; you use CDN + unicast.

Multicast is one of those concepts every networking course teaches but most application developers never use directly. Understanding it helps explain how local-network discovery works (mDNS), how IPTV scales, and why “broadcast” is an outdated term for one-to-many communication. For the related anycast pattern that is relevant to internet apps, see anycast vs unicast; for the IPv6 multicast specifics, IPv6 address types.

Multicast vs Broadcast: One-to-Many Communication