Overview
Unicast and multicast are IP transmission models that differ in how a sender’s data reaches its destinations. Unicast sends a dedicated copy to each individual recipient, while multicast sends a single stream that network devices replicate only where delivery paths actually diverge. The choice shapes bandwidth usage, routing complexity, and how receivers subscribe to traffic.
Comparison Diagram
Comparison Table
| Aspect | Unicast | Multicast |
|---|---|---|
| Addressing model | One-to-one; packet is addressed to a single destination IP | One-to-many; packet is addressed to a shared multicast group IP |
| Sender behavior | Sends a separate copy of the data for each recipient | Sends one copy regardless of how many receivers exist |
| Network replication | No replication; each copy travels its own end-to-end path | Routers/switches replicate the packet only at points where paths diverge |
| Receiver participation | Implicit; determined solely by the destination address | Explicit; hosts must join the group (IGMP/MLD membership) |
| Bandwidth scaling | Grows linearly with the number of receivers | Stays roughly constant on the sender’s link as receivers grow |
| Routing requirements | Standard unicast routing (OSPF, BGP, static routes) | Requires multicast-aware routing (PIM-SM/DM plus IGMP) |
| Delivery reliability | Can run over TCP for guaranteed, ordered delivery | Almost always UDP-based, with no built-in delivery guarantee |
| Typical use cases | Web browsing, file transfer, email, SSH | Live video/audio streaming, market data feeds, routing protocol updates |
Key Differences
- Unicast requires a separate packet copy per receiver; multicast needs only one.
- Multicast pushes replication into the network fabric instead of the sender.
- Multicast receivers must explicitly join a group via IGMP before traffic arrives.
- Unicast bandwidth scales linearly with recipients; multicast stays flat.
- Multicast typically rides over UDP, sacrificing delivery guarantees for efficiency.
When to Use Each
Unicast
- Point-to-Point Transfers: Each recipient needs distinct, personalized data, so a dedicated copy per connection makes sense.
- Interactive Sessions: SSH, HTTPS, and RPC calls require bidirectional, per-client state that only a one-to-one path provides.
- Guaranteed Delivery: Layering TCP over unicast gives retransmission and ordering guarantees multicast can’t offer natively.
Multicast
- Live Streaming to Many: One video feed reaches thousands of viewers without duplicating the stream at the source.
- Financial Market Data: Trading systems need simultaneous, low-latency delivery of the same price feed to many subscribers.
- Routing Protocol Updates: OSPF and EIGRP send hello and update packets to all routers on a segment using well-known multicast addresses.