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Multicast ( English ) referred to in the telecommunications , a communication from one point to a group and is thus a form of multi-point connection . ( See also: group call ), the technique is in accordance with the OSI model in the network layer (Layer 3) is used. Their advantage is that messages can be transmitted to several participants or to a closed group of participants at the same time without the data transmission rate used for this at the sender multiplying by the number of recipients . With multicasting, the sender only needs the same data transmission rate as for a single receiver. In the case of packet- oriented data transmission , the data packets are duplicated at each individual distributor ( router , switch or hub ) on the route .

The difference to broadcast is that with broadcast content is distributed (here: predominantly so-called content ) which - with suitable reception equipment  - anyone can watch, whereas with multicast a registration with the broadcaster is required beforehand .

A special form of multicast is geocast , in which transmission is only carried out in a spatially delimited area.

IP multicast

Multicast is the usual term for IP multicast, which enables packets to be sent efficiently to many recipients at the same time in IP networks. This happens with a special multicast address. In IPv4 , the address range to is reserved, in IPv6 every address starting with FF00 :: / 8 is reserved. In addition, the IGMP or CGMP protocol (Cisco components only) is used for coordination with IPv4 . In IPv6, ICMPv6 takes over the control function.

When transmitting via Ethernet , the IPv4 or IPv6 multicast addresses are mapped to specific pseudo MAC addresses in order to enable the network card to filter for relevant data traffic. The mapping takes place according to the following rules:

IPv4 Multicast to Mac Address German.svg
  • With IPv4, the lowest 23 bits of the IP address are used in the MAC address 01-00-5e-00-00-00, whereby addresses from the range 01-00-5e-00-00-00 to 01- 00-5e-7f-ff-ff. It is consciously accepted that several IPv4 addresses are mapped to the same MAC address (for example and
  • IPv6 multicast addresses are mapped to MAC addresses by inserting the last four bytes of the address into the MAC 33-33-00-00-00-00. Here, too, different multicast addresses are mapped to identical MAC addresses.

The multicast addresses are gaining in importance in IPv6. An IPv6 multicast address has a prefix of FF00 :: / 8 (1111 1111). The second byte of the address FF00 :: / 16 defines the lifetime and the scope. A permanent address has a value of "0", a temporary address has a value of "1" (flag). The validity range of a multicast address varies from node , link , site , organization to global and is indicated with the parameters 1, 2, 5, 8 or E.

Example: Prefix FF02 :: / 16 is a permanent multicast address for a link

Multicasting makes broadcast addresses superfluous. Should z. If, for example, a packet is sent to all network devices in a segment, a special multicast address with the meaning "ALL Nodes" is used.

Multicast is also used in connection with audio and video transmissions. Protocols such as RTP use this mechanism. Routing protocols such as the Routing Information Protocol ( RIP ) version 2 or Open Shortest Path First ( OSPF ) also support multicasting. OSPF uses the addresses or to distribute information.

In addition, multicast is necessary for a functioning AppleTalk network. It is also used in the Service Location Protocol and Multicast DNS as a partial implementation of Zeroconf Multicast . In addition to these protocols currently preferred in the Apple world, multicast is used in Windows systems for SSDP .

Since multicast packets are not processed by most routers in the Internet, subnets with multicast capability are connected to the multicast backbone (MBone) via tunnels .

In the context of Mobile-IP , multicast requires specific support, see RFC 5757 . Particularly in the context of Proxy-Mobile-IP , the IETF has developed easy-to-use and reliable solutions for mobile multicast, see RFC 6224 .


IPsec implements secure communication for point-to-point communication via the Internet Protocol . For encrypted multicast, however, there is a problem with the key exchange , since the recipient determines the authentication and encryption algorithm. With multicast, this has to be done by the sender, as it sends IP packets to multiple recipients using the same algorithm. There are several methods of solving this problem.

Centralized procedures

Distributed proceedings

Other multicast protocols

A critical aspect when handling multicast data traffic is the efficient delivery of the packets to the individual stations on the basis of routing protocols. IP Multicast transmits data from the source to a group of recipients via a tree structure. Different IP multicast routing protocols use different methods to construct this tree. Once this distribution tree has been set up, all data traffic flows through it. Three multicast routing protocols have become established:

IP multicast routing protocols can be divided into two general approaches:

  • the dense mode (“close”), which assumes that the receiving stations in the network are very close together and allow sufficient throughput, and
  • the sparse mode ("sparse"), which assumes that the receivers are distributed very widely over the network and, as in a WAN environment, only have limited bandwidth.

While DVMRP and MOSPF can be assigned to the first class, PIM versions exist for both types.

Internet Relay Chat forms networks that implement a simple TCP -based multicast tree. The PSYC messaging protocol uses a similar principle, with a separate, optimized multicast tree being generated for each chat room or communication channel. For XMPP , consideration is being given to how multicast can be retrofitted, but this is very difficult due to the previous structure. For distributed chat networks it has meanwhile been generally accepted that they cannot be implemented using IP multicast . The use of other multicast protocols is therefore inevitable.

With Multimedia Broadcast Multicast Service , a method has been available for the UMTS mobile radio system since 2005 , which is used to broadcast multimedia content via multicast channels.

Web links


  • RFC 1112 : Host Extensions for IP Multicasting, including mapping of IPv4 multicast addresses to MAC addresses (English)
  • RFC 2464 : Transmission of IPv6 Packets over Ethernet Networks (English)
  • RFC 3180 : GLOP Addressing in 233/8 (English)
  • RFC 4291 : IP Version 6 Addressing Architecture (English)
  • RFC 4608 : Source-Specific Protocol Independent Multicast in 232/8 (English)
  • RFC 5757 Multicast Mobility in Mobile IP Version 6 (MIPv6): Problem Statement and Brief Survey (English)
  • RFC 6224 : Base Deployment for Multicast Listener Support in Proxy Mobile IPv6 (PMIPv6) domains
  • RFC 7046 : A Common API for Transparent Hybrid Multicast
  • RFC 7287 : Mobile Multicast Sender Support in Proxy Mobile IPv6 (PMIPv6) domains.
  • RFC 7411 : Multicast Listener Extensions for Mobile IPv6 (MIPv6) and Proxy Mobile IPv6 (PMIPv6) Fast Handovers.