Multicast

Multicast delivers data from one sender to many receivers that have joined a group address. Routers and switches replicate the packets only where there are subscribers, so the sender transmits one copy.

Why it matters

It is efficient for live video distribution, financial data feeds and service discovery on local networks. IPv4 multicast uses addresses from 224.0.0.0 to 239.255.255.255, and hosts join groups with IGMP (MLD in IPv6). Multicast is rarely routed across the public Internet, so it is mostly used inside managed networks.

If subscribers on a subnet are missing a stream, check that IGMP membership reports reach the router and that the TTL is high enough.

Example

Sender -> 239.1.1.1:5000
Subscribers: A, C, F (joined 239.1.1.1)

The sender emits one stream. The network copies it toward A, C and F only. Unicast would need three separate streams.

Common mistakes

• Expecting multicast to work across the open Internet.
• Using switches without IGMP snooping, which floods multicast to every port.
• Confusing multicast with broadcast, which goes to every host on the segment.

Related

• Broadcast
• Network switch
• UDP
• Router

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