multicast
termDelivery of one IP datagram to a host group, a set of hosts named by a single address, with the network making the copies.
Defined in RFC 1112 (August 1989): a sender transmits once to a group address, in 224.0.0.0/4 for IPv4 or ff00::/8 for IPv6, routers forward it towards networks with members, and hosts report membership with IGMP, or MLD in IPv6. OSPF uses 224.0.0.5 and 224.0.0.6, mDNS 224.0.0.251, and market data feeds have registered groups.
In RFC 1112 a host group is a set of zero or more hosts identified by one destination address. Membership is dynamic: hosts join and leave at any time, may belong to several groups, and need not be members to send. Delivery is best effort, as for unicast. IPv4 groups run from 224.0.0.0 to 239.255.255.255, and in IPv6 a first octet of all ones marks a multicast address. If the application sets no TTL, a multicast datagram defaults to 1, so leaving the local network takes an explicit choice.
IANA reserves 224.0.0.0/24 for routing and other low-level protocols, and multicast routers should not forward it whatever its TTL; 232.0.0.0/8 is kept for source-specific multicast. Routers learn from IGMP which groups have members on each link: they query periodically, and when no reports arrive for a group after some queries they stop forwarding it onto that network.
On Ethernet, RFC 1112 maps the low 23 bits of the group into 01-00-5E-00-00-00, so 32 groups share each MAC address. A switch forwards a frame with a multicast destination out of every port in the forwarding state, wasting bandwidth on hosts that never asked, unless it snoops IGMP or MLD. RFC 4541, an Informational document, still has a snooping switch send 224.0.0.x traffic on all ports and lets it flood unregistered groups by default, and with no querier on the link its entries age out, so the group is flooded or stops, by its policy.
Also known as: ip multicast, host group, multicast group, group address, class d
Sources
- RFC 1112, Host Extensions for IP Multicasting, August 1989: sections 2, 4, 6.1, 6.4 and Appendix I (read 2026-10-19)
- RFC 4291, IP Version 6 Addressing Architecture, February 2006: section 2.7 (read 2026-10-19)
- RFC 4541, Considerations for IGMP and MLD Snooping Switches, May 2006: sections 1, 2.1.1, 2.1.2 and 3 (read 2026-10-19)
- IANA, IPv4 Multicast Address Space registry, last updated 20 August 2026: Note, Local Network Control Block, Internetwork Control Block and Source-Specific Multicast Block (read 2026-10-19)