What Is Multicast?
Multicast — one-to-many delivery: a single stream sent once and received only by devices that joined the group — efficient for video, streaming market data and routing protocols.
How it works
IPv4 multicast uses 224.0.0.0–239.255.255.255. Hosts join groups via IGMP; routers build delivery trees with PIM so each link carries at most one copy of the stream. OSPF itself uses multicast (224.0.0.5) to reach neighbours without disturbing other hosts.
Why it matters
Compare the three delivery modes — unicast (one-to-one), broadcast (one-to-all), multicast (one-to-group) — an evergreen exam and interview item. Multicast design (IGMP/PIM) becomes prominent at CCNP level.
Frequently asked questions
What address range does IPv4 multicast use?
224.0.0.0 through 239.255.255.255 (the old Class D range).
How do hosts join a multicast group?
Via IGMP — they signal interest to the local router, which then delivers that group's traffic to the segment.
Why is multicast efficient for video?
The source sends one stream regardless of audience size; the network duplicates it only where paths diverge.
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