The Four Delivery Modes
| Mode | Destination | Use Case | Example |
|---|---|---|---|
| Unicast | One specific host | Most internet traffic | Loading a webpage, sending an email |
| Multicast | A subscribed group | Live streaming, IPTV, routing protocols | IPTV channel delivery, OSPF hellos |
| Broadcast | All hosts on a subnet | Local network discovery | DHCP discovery, ARP requests |
| Anycast | Nearest node sharing an IP | DNS, CDN, DDoS mitigation | DNS root servers, Cloudflare's 1.1.1.1 |
Unicast: One to One
Unicast is the default and most common IP delivery mode. A unicast packet has a single specific destination IP address, and only that one host receives it. When you open a website, your browser sends a TCP connection request to the server's unicast IP address. The server's response packets are addressed to your device's unicast IP.
The vast majority of internet traffic — web browsing, file downloads, email, video calls — is unicast. Each transfer is a private conversation between two endpoints. Unicast scales well because routers only need to forward each packet once along the path toward its specific destination.
The limitation of unicast becomes apparent when the same content needs to reach many recipients. If a server streams a live video to 10,000 viewers using unicast, it must send 10,000 separate copies of each packet. Multicast and anycast exist to solve this inefficiency in different ways.
Multicast: One to Many (Selective)
Multicast delivers a single packet to a group of receivers who have explicitly opted in — "subscribed" to a multicast group. Routers replicate the stream only at points where the path diverges toward different receivers. This makes multicast extremely efficient for live content delivery: one stream from the source reaches thousands of receivers without sending thousands of separate copies from the server.
IPv4 multicast addresses occupy the 224.0.0.0–239.255.255.255 range (Class D). A receiver joins a multicast group using the IGMP protocol (or MLD for IPv6), which tells the local router to forward that group's traffic to the receiver. Routers use protocols like PIM (Protocol Independent Multicast) to build the multicast distribution tree across the network.
Practical uses of multicast include: IPTV live channel delivery (cable operators use multicast to deliver hundreds of channels efficiently), financial data feeds in trading networks, software deployment in enterprise environments, and routing protocols like OSPF and EIGRP that use well-known multicast addresses to communicate between routers.
Broadcast: One to All (Local)
A broadcast packet is delivered to every host on a network segment simultaneously. IPv4 broadcast uses the subnet's broadcast address — the last address in the subnet range (for example, 192.168.1.255 for a /24 subnet). Every device on that subnet receives the packet regardless of whether it "wanted" to.
Broadcast is confined to a single subnet. Routers do not forward broadcast packets, which is exactly why your network is divided into subnets in the first place — to contain broadcast traffic and prevent it from flooding the entire network.
Broadcast is used for necessary local discovery protocols. When a device first connects to a network and does not yet know its router's IP, it sends a DHCP Discover packet to the broadcast address — the only way to reach a DHCP server whose address is unknown. ARP (Address Resolution Protocol) uses broadcast to ask "who has IP address X?" so devices can find each other's MAC addresses on a local network.
IPv6 eliminated broadcast entirely, replacing it with targeted multicast. This reduces unnecessary processing on every device — in IPv4, every host must examine every broadcast packet, even if it has nothing to do with that host.
Anycast: One to Nearest
Anycast assigns the same IP address to multiple servers in different geographic locations. When a packet is sent to an anycast address, the routing system delivers it to whichever server sharing that address is topologically nearest (fewest hops or lowest routing cost). The sender has no knowledge that anycast is involved — it sends to a single IP address and receives a response from whichever node answered.
Anycast is made possible by BGP (Border Gateway Protocol). Each anycast node advertises the same IP prefix to its neighboring routers, and BGP selects the best path to each node based on routing metrics. Users in different parts of the world naturally reach different physical servers when querying the same anycast IP.
The DNS root server system is the classic anycast deployment. All 13 root server addresses (such as 198.41.0.4 for the A root) are served by hundreds of physical servers worldwide. When your resolver queries an anycast root server, it automatically reaches a nearby instance rather than one on another continent. Cloudflare's 1.1.1.1 DNS resolver also uses anycast — requests route to Cloudflare's nearest data center, keeping latency low globally.
Why These Distinctions Matter for Networks
Choosing the wrong delivery mode wastes bandwidth and creates scalability problems. A video streaming service that uses unicast for live broadcasts must provision server capacity proportional to viewer count. One that uses multicast (where the network supports it) or anycast-based CDN can serve orders of magnitude more viewers with the same server resources.
For network engineers, understanding delivery modes is essential for designing efficient networks — controlling where broadcast domains end, setting up multicast routing, and configuring anycast for high-availability services. For end users, these concepts explain why services like DNS feel instantaneous even from the other side of the world, and why live TV on some providers works with much lower server costs than you might expect.
Frequently Asked Questions
What is unicast traffic?
Unicast traffic is a one-to-one packet delivery — each packet has a single specific destination host and only that host receives it. Web browsing, email, and file downloads are all unicast.
What is multicast used for?
Multicast delivers one packet stream to multiple subscribed receivers simultaneously. It is used for IPTV live channels, financial data feeds, routing protocol communications, and enterprise software distribution where the same data must reach many receivers efficiently.
What is a broadcast address?
A broadcast address targets all hosts on a local subnet simultaneously. In IPv4, it is the last address in the subnet range (e.g., 192.168.1.255 for a /24). Routers do not forward broadcasts — they stay within a subnet. DHCP discovery and ARP use broadcast for local device discovery.
What is anycast used for?
Anycast routes packets to the nearest physical node sharing an IP address. It is used by DNS root servers, CDN edge networks, and DDoS mitigation services to automatically route users to the closest available server without any client-side configuration.
Does IPv6 support broadcast?
No. IPv6 eliminated broadcast entirely and replaced it with targeted multicast. ARP was replaced by Neighbor Discovery Protocol (NDP) which uses multicast. This reduces unnecessary traffic on networks where every broadcast packet previously interrupted every host.
What address range does IPv4 multicast use?
IPv4 multicast addresses are in the 224.0.0.0 to 239.255.255.255 range (Class D). Addresses in 224.0.0.0/24 are link-local (router-only, not forwarded). The range 239.0.0.0/8 is reserved for private use within organizations.