Unicast sends a packet to one specific device. Multicast sends one packet to a group of devices that joined that group, such as Bonjour's 224.0.0.251. Broadcast sends to every device on the local network, such as 255.255.255.255. Routers don't forward broadcasts.
How it works
Unicast
Normal one-to-one traffic: web pages, downloads, SSH. Almost everything you do is unicast.
Multicast
The sender transmits once to a group address (224.0.0.0/4 in IPv4, ff00::/8 in IPv6). Only devices that joined the group process it, and switches and routers that support multicast deliver copies only where needed. It saves bandwidth when many receivers want the same data, as with IPTV or stock tickers. On home networks, the main users are Bonjour/mDNS (224.0.0.251, port 5353) and SSDP/UPnP (239.255.255.250).
Broadcast
Sent to every device on the subnet: 255.255.255.255 (limited broadcast) or the subnet's broadcast address (e.g. 192.168.1.255). Used by DHCP discovery and ARP requests. Routers don't forward broadcasts, which is one reason networks are split into subnets. IPv6 has no broadcast and uses multicast instead.
Anycast
One address announced from many places, with each packet going to the nearest one. Public DNS resolvers like 1.1.1.1 and CDNs use it.
Side by side
| Unicast | Multicast | Broadcast | |
|---|---|---|---|
| Receivers | One | Members of a group | Everyone on the LAN |
| Example address | 192.168.1.40 | 224.0.0.251 | 255.255.255.255 |
| Crosses routers | Yes | If configured | No |
| Example use | Web, SSH, downloads | Bonjour, IPTV | DHCP, ARP |
Examples
- Your Mac finding a printer: a multicast mDNS query to 224.0.0.251:5353, and the printer replies.
- Your Mac joining Wi-Fi: a DHCP Discover broadcast to 255.255.255.255.
- Loading a web page: unicast between your Mac and the server.
On a Mac
netstat -an -p udp | grep 5353 shows the mDNS sockets macOS keeps open for Bonjour. netstat -g lists the multicast groups your interfaces have joined.
See it in the app
LightAtom groups unconnected UDP sockets, which is where multicast and broadcast traffic like mDNSResponder's Bonjour queries shows up, into one row per app.
Frequently asked questions
What is the difference between multicast and broadcast?
Broadcast goes to every device on the local network. Multicast goes only to devices that joined a specific group, and can be routed between networks.
What is multicast used for?
On home networks mainly device discovery: Bonjour/mDNS and UPnP. In larger networks, video distribution, market data and routing protocols.
Does IPv6 use broadcast?
No. IPv6 replaced broadcast with multicast groups, such as ff02::1 for all nodes on a link.
What is a broadcast address?
The last address of a subnet (like 192.168.1.255 for 192.168.1.0/24), which reaches every device on it.