DDI glossary

What is multicast?

Multicast is an IP addressing and delivery method that sends one stream of packets to a group of receivers at the same time. It uses reserved group addresses, and hosts opt in by joining a group, so routers replicate traffic only where members exist.

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Multicast is an IP addressing and delivery method that sends a single stream of packets to multiple receivers simultaneously by using group addresses, enabling efficient distribution of data to many hosts without requiring separate copies from the source. The technique uses reserved address ranges (224.0.0.0/4 for IPv4 and ff00::/8 for IPv6) and relies on membership protocols like IGMP and MLD to allow hosts to join groups, with routers replicating traffic only where group members exist. Poor multicast management causes difficult-to-diagnose network problems such as VLAN flooding, blocked membership messages, and failed routing adjacencies, but proper configuration of switch snooping, group address allocation, and traffic filtering ensures reliable delivery for critical functions like OSPF routing and IPv6 neighbor discovery.

  • Multicast typically operates over UDP rather than TCP because TCP requires point-to-point connections between two endpoints, which cannot accommodate one sender delivering to many receivers simultaneously.
  • IPv4 multicast addresses range from 224.0.0.0 through 239.255.255.255 in the 224.0.0.0/4 block, while IPv6 multicast addresses begin with ff00::/8 and include flag and scope fields that control how far traffic propagates across network domains.
  • Common multicast deployment failures include switch flooding when IGMP or MLD snooping is disabled, reverse path check failures from asymmetric routing, TTL scope limitations preventing cross-router delivery, and firewall rules that block group addresses or membership signaling protocols.

Multicast at a glance

AttributeValue
Full nameMulticast
Categoryaddressing and packet delivery method
Defined inRFC 1112 for IPv4 host group membership, RFC 3376 for IGMPv3, RFC 4291 for IPv6 addressing, RFC 3810 for MLDv2
Primary useDelivering the same data to many receivers without sending a separate copy to each, such as routing protocol updates, media distribution and service discovery
Related termsUnicast, Broadcast, Anycast, IGMP, OSPF, IPv6

How multicast works

Multicast separates the sender from the receivers by using a group address. The sender transmits once to that address, and the network delivers copies only to hosts that have asked to receive them.

  1. Group address assignment: A multicast application is assigned a group address. IPv4 multicast uses the 224.0.0.0/4 range, and IPv6 multicast uses addresses beginning with ff00::/8.
  2. Group membership: A receiver signals interest in a group to its local router. IPv4 hosts use the Internet Group Management Protocol, and IPv6 hosts use Multicast Listener Discovery.
  3. Distribution tree: Multicast routing protocols, such as Protocol Independent Multicast, build a tree of paths from the source toward the routers that have members.
  4. Packet replication: The sender transmits each packet once. Routers and switches copy it only at points where the tree branches toward members.
  5. Leaving the group: When a host leaves or stops responding to membership queries, the router prunes that branch so traffic stops flowing to it.

Multicast delivery is typically sent over UDP and is not retransmitted by the network, so applications must tolerate loss or add their own recovery. Some addresses are reserved for infrastructure use. For example, OSPF uses 224.0.0.5 and 224.0.0.6 on IPv4, and IPv6 relies on multicast for neighbor discovery.

Why multicast matters in enterprise networks

Multicast matters because sending a separate unicast copy to every receiver scales poorly. A source serving thousands of receivers would need bandwidth proportional to the audience, while multicast keeps the load on the source and shared links roughly constant. Many core network functions also depend on it. OSPF sends hello packets to 224.0.0.5 and 224.0.0.6 on IPv4, and IPv6 removed broadcast, so functions such as neighbor discovery and DHCPv6 server discovery use multicast groups instead.

Poor multicast management causes problems that are hard to trace. If switches do not snoop group membership, multicast frames flood every port in a VLAN, which can saturate access links. Mismatched group addresses, blocked membership messages or a missing rendezvous point in sparse-mode designs leave receivers without traffic while unicast appears healthy. Routing adjacencies that depend on multicast can drop if filters or access lists block the reserved groups.

Security and visibility also need attention. Any host can usually join a group or send to it unless the network restricts it, so unplanned sources can inject traffic or receivers can eavesdrop. Reviewing group address allocation, limiting who can source traffic, and monitoring membership and flow counters help keep multicast predictable.

Multicast vs Unicast (routing)

Unicast delivers each packet to a single destination address, while multicast delivers one packet stream to every host that has joined a group address. What is Unicast?

AttributeMulticastUnicast (routing)
DestinationA group address that many hosts can joinOne specific host address
Source bandwidth for many receiversRoughly constant, because routers replicate packetsGrows with each added receiver
Receiver signalingHosts join a group with IGMP or MLDNo group membership is needed
Transport protocolTypically UDP, since TCP needs a single peerTCP or UDP

Multicast vs Broadcast routing

Broadcast sends packets to every host on a network segment, while multicast sends them only to hosts that joined the group. What is Broadcast routing?

AttributeMulticastBroadcast routing
RecipientsOnly hosts that joined the groupAll hosts in the broadcast domain
Opt-inRequired through IGMP or MLDNot possible; every host processes the packet
IPv6 supportUsed in IPv6 for functions broadcast once handledNot present in IPv6

Multicast vs Anycast

Anycast delivers a packet to the single nearest host that shares an address, while multicast delivers it to all members of a group. What is Anycast?

AttributeMulticastAnycast
DeliveryTo all group membersTo one nearest instance
Typical useStreaming, routing protocol hellos, discoveryResilient services such as DNS
Address typeReserved group rangesOrdinary unicast addresses announced from several locations

Common multicast issues

Without IGMP snooping or MLD snooping, a Layer 2 switch treats multicast like broadcast and floods it to every port. Symptoms include saturated access links and slow hosts that never joined the group.

When snooping is on but no querier exists in the VLAN, membership entries age out. Streams then work for a few minutes and stop, a pattern that repeats until the hosts send new reports.

Protocol Independent Multicast checks that traffic arrives on the interface used to reach the source. Asymmetric routing or a missing route to the source makes routers drop packets, so receivers see no stream.

Multicast packets sent with a TTL of 1 never cross a router. An application that works inside one subnet then fails between sites until the TTL or hop limit is raised.

Firewall rules often permit unicast but drop group addresses or IGMP and MLD messages. Routing adjacencies that rely on multicast hellos, such as OSPF, then fail to form.

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Multicast FAQ

Related terms

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Unicast (routing)

One-to-one transmission from one point in the network to another point with one sender and one receiver, each identified by a network address.

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Broadcast routing

Packets are sent to all nodes even if they do not want it.

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Anycast

A traffic routing algorithm used for the speedy delivery of website content that advertises individual IP addresses on multiple nodes.

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Open Shortest Path First (OSPF)

A routing protocol for IP networks that uses a link state routing algorithm and falls into the group of interior gateway protocols, operating within a single autonomous system.

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IPv6 (Internet Protocol version 6)

The most recent version of Internet Protocol, which uses 128-bit address space; there are 340 undecillion IPv6 addresses.

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IPv4 (Internet Protocol version 4)

The fourth version of Internet Protocol that uses 32-bit addresses to route most of today’s internet traffic; the global supply of IPv4 addresses is exhausted. Learn More

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User Datagram Protocol (UDP)

An alternative protocol to TCP used primarily for establishing low-latency and loss-tolerating connections between applications on the internet.

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Geocast

The delivery of information to a group of destinations in a network identified by their geographical locations; a specialized form of multicast addressing.

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