Multicast address

A multicast address is a logical identifier for a group of hosts in a computer network, that are available to process datagrams or frames intended to be multicast for a designated network service. Multicast addressing can be used in the Link Layer (Layer 2 in the OSI model), such as Ethernet multicast, and at the Internet Layer (Layer 3 for OSI) for Internet Protocol Version 4 (IPv4) or Version 6 (IPv6) multicast.

IPv4

IPv4 multicast addresses are defined by the leading address bits of 1110, originating from the classful network design of the early Internet when this group of addresses was designated as Class D. The Classless Inter-Domain Routing (CIDR) prefix of this group is 224.0.0.0/4. The group includes the addresses from 224.0.0.0 to 239.255.255.255. Address assignments from within this range are specified in RFC 5771, an Internet Engineering Task Force (IETF) Best Current Practice document (BCP 51).

The following table is a list of notable well-known IPv4 addresses that are reserved for IP multicasting and that are registered with the Internet Assigned Numbers Authority (IANA).^[1]

IP multicast address	Description
224.0.0.0	Base address (reserved)
224.0.0.1	The All Hosts multicast group addresses all hosts on the same network segment.
224.0.0.2	The All Routers multicast group addresses all routers on the same network segment.
224.0.0.4	This address is used in the Distance Vector Multicast Routing Protocol (DVMRP) to address multicast routers.
224.0.0.5	The Open Shortest Path First (OSPF) All OSPF Routers address is used to send Hello packets to all OSPF routers on a network segment.
224.0.0.6	The OSPF All D Routers address is used to send OSPF routing information to designated routers on a network segment.
224.0.0.9	The Routing Information Protocol (RIP) version 2 group address is used to send routing information to all RIP2-aware routers on a network segment.
224.0.0.10	The Enhanced Interior Gateway Routing Protocol (EIGRP) group address is used to send routing information to all EIGRP routers on a network segment.
224.0.0.13	Protocol Independent Multicast (PIM) Version 2
224.0.0.18	Virtual Router Redundancy Protocol (VRRP)
224.0.0.19 - 21	IS-IS over IP
224.0.0.22	Internet Group Management Protocol (IGMP) Version 3
224.0.0.102	Hot Standby Router Protocol version 2 (HSRPv2) / Gateway Load Balancing Protocol (GLBP)
224.0.0.107	Precision Time Protocol version 2 peer delay measurement messaging
224.0.0.251	Multicast DNS (mDNS) address
224.0.0.252	Link-local Multicast Name Resolution (LLMNR) address
224.0.1.1	Network Time Protocol clients listen on this address for protocol messages when operating in multicast mode.
224.0.1.39	The Cisco multicast router AUTO-RP-ANNOUNCE address is used by RP mapping agents to listen for candidate announcements.
224.0.1.40	The Cisco multicast router AUTO-RP-DISCOVERY address is the destination address for messages from the RP mapping agent to discover candidates.
224.0.1.41	H.323 Gatekeeper discovery address
224.0.1.129 - 132	Precision Time Protocol version 1 time announcements
224.0.1.129	Precision Time Protocol version 2 time announcements

Local subnetwork

Addresses in the range 224.0.0.0 to 224.0.0.255 are individually assigned by IANA and designated for multicasting on the local subnetwork only. For example, the Routing Information Protocol (RIPv2) uses 224.0.0.9, Open Shortest Path First (OSPF) uses 224.0.0.5 & 224.0.0.6, and Zeroconf mDNS uses 224.0.0.251. Routers must not forward these messages outside the subnet in which they originate.

Internetwork control block

Addresses in the range 224.0.1.0 to 224.0.1.255 are individually assigned by IANA and designated the Internetwork Control Block. It is used for traffic that must be routed through the public Internet, such as for applications of the Network Time Protocol (224.0.1.1).

AD-HOC block

Addresses in the ranges 224.0.2.0 to 224.0.255.255, 224.3.0.0 to 224.4.255.255 and 233.252.0.0 to 233.255.255.255 are individually assigned by IANA and designated the AD-HOC block. These addresses are globally routed and are used for applications that don't fit either of the previously described purposes.^[2]

Source-specific multicast

The 232.0.0.0/8 block is reserved for use by source-specific multicast.

GLOP addressing

The 233.0.0.0/8 range was originally assigned by RFC 2770 as an experimental, public statically assigned multicast address space for publishers and Internet service providers that wished to source content on the Internet. The allocation method is termed GLOP addressing and provides implementers a block of 255 addresses that is determined by their 16-bit autonomous system number (ASN) allocation. In a nutshell, the middle two octets of this block are formed from assigned ASNs, giving any operator assigned an ASN 256 globally unique multicast group addresses. The method is not applicable to the newer 32-bit extension AS numbers. RFC 3180, superseding RFC 2770, envisioned the use of the range for many-to-many multicast applications. This block has been one of the most successful multicast addressing schemes.^{[citation needed]} Unfortunately, with only 256 multicast addresses available to each autonomous system, GLOP is not adequate for large-scale broadcasters.^[3]

Unicast-Prefix-Based IPv4 Multicast addresses

The 234.0.0.0/8 range is assigned by RFC 6034 as a range of global IPv4 multicast address space provided to each organization that has /24 or larger globally routed unicast address space allocated. A resulting advantage over GLOP is that the mechanisms in IPv4 and IPv6 become more similar.

Administratively scoped addresses

The 239.0.0.0/8 range is assigned by RFC 2365 as a locally administered address space with local or organizational scope. It may be used by anyone, without concern for address collisions, for private multicast domains.

The administratively scoped address block is further subdivided.

1. 239.255.0.0/16 is designated for local scope. Any scope boundary is a boundary for local scope. Local scope address space may be extended downwards if necessary (i.e. to 239.254.0.0/16, 239.253.0.0/16, etc.).^[4]
2. 239.192.0.0/14 is designated for organizational local scope. This space may be extended to 239.0.0.0/10, 239.64.0.0/10 and 239.128.0.0/10 if necessary.^[5]

Within each scope, the top 256 addresses are reserved for relative assignments. For example, the topmost relative assignment address is used by the Session Announcement Protocol.^[6]

IPv6

Multicast addresses in IPv6 have the prefix ff00::/8. IPv6 multicast addresses are generally formed from four bit groups, illustrated as follows:

General multicast address format

Bits	8	4	4	112
Field	prefix	flgs	sc	group ID

The prefix holds the binary value 11111111 for any multicast address. Currently, 3 of the 4 flag bits in the flgs field are defined;^[7] the most-significant flag bit is reserved for future use. The other three flags are known as R, P and T.

Multicast address flags^[8]

Flag	0	1
R (Rendezvous)[9]	Rendezvous point not embedded	Rendezvous point embedded
P (Prefix)[10]	Without prefix information	Address based on network prefix
T (Transient)[11]	Well-known multicast address	Dynamically assigned multicast address

Similar to unicast addresses, the prefix of IPv6 multicast addresses specifies their scope, however, the set of possible scopes is different. The 4-bit sc (or scope) field (bits 12 to 15) is used to indicate where the address is valid and unique.

Multicast address scope

IPv6 address[note 1]	IPv4 equivalent[12]	Scope	Purpose
ff00::/16-ff0f::/16		Reserved
ffx1::/16	127.0.0.0/8	Interface-local	Packets with this destination address may not be sent over any network link, but must remain within the current node; this is the multicast equivalent of the unicast loopback address.
ffx2::/16	224.0.0.0/24	Link-local	Packets with this destination address may not be routed anywhere.
ffx3::/16	239.255.0.0/16	IPv4 local scope
ffx4::/16		Admin-local	The smallest scope that must be administratively configured.
ffx5::/16		Site-local	Restricted to the local physical network.
ffx8::/16	239.192.0.0/14	Organization-local	Restricted to networks used by the organization administering the local network. (For example, these addresses might be used over VPNs; when packets for this group are routed over the public internet (where these addresses are not valid), they would have to be encapsulated in some other protocol.)
ffxe::/16	224.0.1.0-238.255.255.255	Global scope	Eligible to be routed over the public internet.

The service is identified in the 112-bit Group ID field. For example, if ff02::101 refers to all Network Time Protocol (NTP) servers on the local network segment, then ff08::101 refers to all NTP servers in an organization's networks. The Group ID field may be further divided for special multicast address types.

The following table is a partial list of well-known IPv6 multicast addresses that are registered with the Internet Assigned Numbers Authority (IANA).^[13]

Well-known IPv6 multicast addresses

Address	Description
ff02::1	All nodes on the local network segment
ff02::2	All routers on the local network segment
ff02::5	OSPFv3 AllSPF routers
ff02::6	OSPFv3 AllDR routers
ff02::9	RIP routers
ff02::a	EIGRP routers
ff02::d	PIM routers
ff02::16	MLDv2 reports (defined in RFC 3810)
ff02::1:2	All DHCP servers and relay agents on the local network site (defined in RFC 3315)
ff05::1:3	All DHCP servers on the local network site (defined in RFC 3315)
ff0x::fb	Multicast DNS
ff0x::101	Network Time Protocol
ff0x::108	Network Information Service
ff0x::114	Used for experiments

Ethernet

Ethernet frames with a value of 1 in the least-significant bit of the first octet^{[note 2]} of the destination address are treated as multicast frames and are flooded to all points on the network. While frames with ones in all bits of the destination address (FF:FF:FF:FF:FF:FF) are sometimes referred to as broadcasts, Ethernet network equipment generally does not distinguish between multicast and broadcast frames. Modern Ethernet controllers filter received packets to reduce CPU load, by looking up the hash of a multicast destination address in a table, initialized by software, which controls whether a multicast packet is dropped or fully received.

Some well known Ethernet multicast addresses^[14]

Ethernet multicast address	Type Field	Usage
01-00-0C-CC-CC-CC	0x0802	CDP (Cisco Discovery Protocol), VTP (VLAN Trunking Protocol)
01-00-0C-CC-CC-CD	0x0802	Cisco Shared Spanning Tree Protocol Address
01-80-C2-00-00-00	0x0802	Spanning Tree Protocol (for bridges) IEEE 802.1D
01-80-C2-00-00-08	0x0802	Spanning Tree Protocol (for provider bridges) IEEE 802.1AD
01-80-C2-00-00-02	0x8809	Ethernet OAM Protocol IEEE 802.3ah
01-00-5E-xx-xx-xx	0x0800	IPv4 Multicast (RFC 1112)
33-33-xx-xx-xx-xx	0x86DD	IPv6 Multicast (RFC 2464)

See also

• Broadcast address

Notes

1. ^ x is a place holder indicating that the value of the flags field is unimportant in the current discussion.
2. ^ On Ethernet, the least-significant bit of an octet is the first to be transmitted. A multicast is indicated by the first transmitted bit of the destination address being 1.

References

1. ^ IANA IP multicast addresses assignments. Internet Assigned Numbers Authority .
2. ^ RFC 5771 Section 6.
3. ^ "Frequently Asked Questions (FAQ) File for Multicasting". Multicast Tech. http://www.multicasttech.com/faq/. Retrieved 2010-11-18. 
4. ^ RFC 2365 section 6.1.
5. ^ RFC 2365 section 6.2.
6. ^ RFC 2365 section 9
7. ^ Hinden, R.; Deering, S. (February 2006) IP Version 6 Addressing Architecture, IETF, RFC 4291.
8. ^ Silvia Hagen (May 2006). IPv6 Essentials (Second ed.). O'Reilly. ISBN 978-0596100582. 
9. ^ RFC 3956
10. ^ RFC 3306
11. ^ RFC 4291
12. ^ RFC 2365 section 8.
13. ^ IANA Internet Protocol Version 6 Multicast Addresses. Internet Assigned Numbers Authority.
14. ^ Patton, Michael A. et. al.. "Multicast (including Broadcast) Addresses". cavebear.com. Karl Auerbach. http://www.cavebear.com/archive/cavebear/Ethernet/multicast.html.