Unicast sends an individual flow
With unicast, each viewer requests data addressed to that viewer’s connection. This maps naturally to on-demand playback because people start at different times and choose different positions. HTTP-based HLS and DASH delivery is normally unicast, with caches and CDNs reducing repeated work near viewers.
Multicast distributes a shared flow inside a capable network
IP multicast sends traffic to a group address. Network equipment replicates packets only where paths branch toward receivers that joined the group. A managed IPTV operator can therefore carry one live channel through a network segment instead of one identical copy per household.
Receivers use group-management mechanisms such as IGMP on IPv4 networks. Routers and switches must be configured for multicast; entering a multicast URL does not make an arbitrary public Internet path support it.
Why multicast is not the default for web video
- Internet routers do not provide general end-to-end multicast service to ordinary websites
- Home gateways, Wi-Fi and VPN paths may handle multicast inconsistently
- Per-viewer pause, seek, entitlement and advertising are simpler with unicast
- HTTP delivery works with browsers, CDNs, caches and existing firewall rules
Channel changing reveals the architectural difference
A multicast receiver leaves one group and joins another when the viewer changes channel. A unicast HTTP player stops requesting segments from one stream and starts a different session. Join time, key-frame availability, buffer strategy and network delay all influence how fast the new channel appears.
VLC can demonstrate both models on a local network
VideoLAN documents UDP and RTP inputs for unicast and multicast. These are useful for authorised laboratory or enterprise networks. Public IPTV web playback more commonly uses HTTPS HLS because browsers cannot directly join arbitrary IP multicast groups.