Broadcasting vs Multicasting — Which Is Right for Live Video Distribution?

Ask ten network engineers to define broadcasting versus multicasting and you will get ten slightly different answers, usually shaped by whatever world they came up in — television playout, enterprise IP, or financial market-data feeds. The confusion is fair, because the same two words carry different weight depending on which layer of the stack you are standing on. So let us pin the terms down where it actually matters, which is how packets move across a network, and then follow the thread out to live video on the public internet — where the choice between these models decides how you distribute a feed at all.

The short version: broadcast sends one stream to every device on a network segment, multicast sends one stream to a subscribed group of devices, and unicast sends one stream to exactly one device. Three delivery models, three very different reach and cost profiles. The interesting part is not the definitions — it is where each one works, and where it flatly does not.

The three delivery models

Broadcast: one to everyone on the segment

In networking terms, a broadcast is a packet addressed to all hosts on a local segment at once. Send to the broadcast address and every device in that broadcast domain receives and processes the frame, whether it wanted the traffic or not. This is how a machine that knows nothing about its neighbours bootstraps itself: ARP shouts "who has this IP?" to the whole subnet, and a DHCP client broadcasts to find any server that will answer.

The limitation is baked into the design. A broadcast never leaves its subnet — routers deliberately do not forward it — so its reach is a single LAN segment. And because every host has to inspect every broadcast frame, flooding a network with them is a classic way to bring it to its knees. Broadcast is a local, indiscriminate tool, which is exactly why IPv6 quietly retired it in favour of multicast. (This is also, incidentally, where the everyday sense of broadcast media comes from: radio and television transmit one signal that anyone in range can pick up. Same one-to-all idea, just over the airwaves instead of a wire.)

Multicast: one to a subscribed group

Multicast is the clever middle ground, and the direct answer to "what is multicasting": instead of one address per receiver as in unicast, or one address for everyone as in broadcast, traffic is sent to a single group address — in IPv4, a Class D address in the 224.0.0.0 to 239.255.255.255 range — and only the hosts that have explicitly joined that group receive it. On a local network, hosts announce their interest using IGMP (Internet Group Management Protocol); between routers, the network builds an efficient distribution tree using PIM (Protocol Independent Multicast).

The efficiency is the whole point. The source emits a single stream. The network carries just one copy down any given link and only replicates a packet at the points where the paths to different receivers diverge. Add a thousand subscribers on the same branch and the sender's output does not change by a single bit. That is why multicast underpins IPTV, live IP-based broadcast plants, and the market-data fire-hoses feeding trading floors — anywhere a large number of receivers on a managed network need the identical stream at the same instant.

Unicast: one to one

Unicast is the default the entire internet runs on: one sender, one receiver, one dedicated flow. Every web page you load and nearly every video you stream arrives by unicast. It is simple, universally routable, and it scales predictably — which is also its weakness for one-to-many delivery. Ten identical receivers mean ten identical streams leaving the source; a hundred mean a hundred. Bandwidth grows linearly with the audience, because the network is doing no clever replication on your behalf.

Broadcast vs multicast vs unicast at a glance

Why multicast is brilliant inside a network — and missing from the internet

Here is the insight that trips up nearly everyone arriving at this from the streaming side: IP multicast essentially does not work across the public internet. It is superb technology, but it is technology for networks under a single administrative roof.

The reason is part technical and mostly commercial. Multicast requires every router on the path to participate — to hold group state, run PIM, and honour IGMP joins. Inside one operator's network that is entirely achievable, and carriers do it every day to deliver IPTV to millions of set-top boxes. Between operators it falls apart. Inter-domain multicast — the machinery of MSDP and multicast BGP — never saw meaningful commercial adoption. There is no business model for one ISP to carry another's multicast groups, no clean way to police who joins what, and no appetite to hold that state for the open internet. The 1990s MBONE experiment was the high-water mark, and it receded. In practice, the moment your traffic crosses an ISP boundary, assume multicast is off the table.

None of this is a knock on multicast. Inside a managed media network it is the right answer, and serious vendors have built serious infrastructure around it. Net Insight, for one, has earned real authority in IP-based broadcast transport and media networking, where deterministic multicast distribution and precise timing are exactly what a facility needs. If you own the network end to end, multicast is a gift. The problem is that most people distributing live video today do not own the network between themselves and their destinations. The public internet sits in the middle, and it does not speak multicast.

One-to-many live video over a network you don't control

So how do you get multicast-style one-to-many distribution when the path runs across the open internet, where multicast cannot reach? You go back to the one model that always routes — unicast — and you solve reliability and fan-out as two separate problems.

Reliability comes from a modern unicast transport. SRT (Secure Reliable Transport), the open protocol from Haivision, runs over UDP and uses ARQ retransmission inside a bounded latency window, plus AES encryption, to carry broadcast-grade video across lossy, unpredictable internet paths — the job that plain TCP-based streaming does badly. SRT gives you a clean, secure feed from point A to point B. What it does not do on its own is turn that one feed into many.

The fan-out is the second half, and it is where a distribution service earns its keep. You take the single SRT stream and replicate it into as many independent unicast copies as you have destinations, then deliver each copy to its own endpoint. Functionally you have reproduced what multicast does — one input, many identical receivers — but you have done it with unicast plumbing that actually works over the internet, without asking a single ISP to route a multicast group.

How SRT Cloud does the fan-out

This is precisely the problem SRT Cloud is built for. You send one live SRT input; SRT Cloud produces unlimited bit-exact, 1:1 copies and delivers each one to a taker — a downstream destination such as a broadcaster, telco, affiliate, CDN, or satellite operator. There is no transcoding and no re-encoding: every output is frame-for-frame identical to the source, so each taker receives exactly the feed you sent. Real takers on the platform today include FreeSat, Pyur, Eutelsat, SES, Etisalat, Swisscom, UIG, and KiwiSat.

Think of it as multicast's one-to-many reach delivered over the open internet with none of multicast's network prerequisites. There is no hardware to rack, no media server to run, and no multicast-capable path to negotiate with anyone's carrier — just one SRT feed in and as many exact copies out as you need. Pricing follows the same one-to-one logic: €99 per output per month, one license per SRT output, after a free trial and with no long-term commitment, so your cost tracks the number of destinations you actually serve.

Broadcast, multicast, and unicast are the three ways a network can move a packet. Multicast wins inside the walls of a managed network; unicast is the only one that reaches the whole internet. Live one-to-many distribution over that internet is really a unicast problem wearing a multicast costume — reliable transport plus disciplined fan-out — and that is the job SRT Cloud quietly does for every feed it carries.