Multicast address explained

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 most-significant bit pattern of 1110. This originates from the classful network design of the early Internet when this group of addresses was designated as Class D. The CIDR notation for this group is .^[1] The group includes the addresses from to .

The address range is divided into blocks each assigned a specific purpose or behavior.

IP multicast address range	Description	Routable
224.0.0.0 to 224.0.0.255	Local subnetwork
224.0.1.0 to 224.0.1.255	Internetwork control
224.0.2.0 to 224.0.255.255	AD-HOC block 1^[2]
224.1.0.0 to 224.1.255.255	Reserved
224.2.0.0 to 224.2.255.255	SDP/SAP block^[3]
224.3.0.0 to 224.4.255.255	AD-HOC block 2^[4]
225.0.0.0 to 231.255.255.255	Reserved
232.0.0.0 to 232.255.255.255	Source-specific multicast
233.0.0.0 to 233.251.255.255	GLOP addressing^[5]
233.252.0.0 to 233.255.255.255	AD-HOC block 3^[6]
234.0.0.0 to 234.255.255.255	Unicast-prefix-based^[7]
235.0.0.0 to 238.255.255.255	Reserved
239.0.0.0 to 239.255.255.255	Administratively scoped

Local subnetwork

Addresses in the range of to are individually assigned by IANA and designated for multicasting on the local subnetwork only. For example, the Routing Information Protocol (RIPv2) uses, Open Shortest Path First (OSPF) uses and, and Multicast DNS uses . Routers must not forward these messages outside the subnet from which they originate.

Internetwork control block

Addresses in the range to are individually assigned by IANA and designated as the internetwork control block. This block of addresses is used for traffic that must be routed through the public Internet, such as for applications of the Network Time Protocol using .

AD-HOC block

Addresses in three separate blocks are not individually assigned by IANA. These addresses are globally routed and are used for applications that don't fit either of the previously described purposes.

SDP/SAP block

Addresses in the reserved range are not individually assigned by IANA. Fallen out of use for security considerations, experimental Session Announcement Protocol was the primary means of supplying addresses through Session Description Protocol, which now is mostly used in the establishment of private sessions.

Source-specific multicast

The (IPv4) and (IPv6) blocks are reserved for use by source-specific multicast.

GLOP^[8]

The range was originally assigned 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.^[9] The method is not applicable to the newer 32-bit ASNs. In, the IETF envisioned a broader use of the range for many-to-many multicast applications. Unfortunately, with only 256 multicast addresses available to each autonomous system, GLOP is not adequate for large-scale broadcasters.

Unicast-prefix-based

The range is assigned as a range of global IPv4 multicast address space provided to each organization that has or larger globally routed unicast address space allocated; one multicast address is reserved per of unicast space. A resulting advantage over GLOP is that the unicast-prefix mechanism resembles the unicast-prefix capabilities of IPv6.

Administratively scoped

The range is assigned for private use within an organization. Packets destined to administratively scoped IPv4 multicast addresses do not cross administratively defined organizational boundaries, and administratively scoped IPv4 multicast addresses are locally assigned and do not have to be globally unique. The range may be structured to be loosely similar to the scoped IPv6 multicast address.

Ethernet-specific

In support of link-local multicasts which do not use IGMP, any IPv4 multicast address that falls within the and ranges will be broadcast to all ports on many Ethernet switches, even if IGMP snooping is enabled, so addresses within these ranges should be avoided on Ethernet networks where the functionality of IGMP snooping is desired.^[10]

Notable IPv4 multicast addresses

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).

IP multicast address	Description	Routable
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 Designated Routers (DR) 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 (PTP) 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
224.0.0.253	Teredo tunneling client discovery address
224.0.1.1	Network Time Protocol clients listen on this address for protocol messages when operating in multicast mode.
224.0.1.22	Service Location Protocol version 1 general
224.0.1.35	Service Location Protocol version 1 directory agent
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 (PTP) version 1 messages (Sync, Announce, etc.) except peer delay measurement
224.0.1.129	Precision Time Protocol (PTP) version 2 messages (Sync, Announce, etc.) except peer delay measurement
224.0.23.12	KNXnet/IP discovery^[11]
239.255.255.250	Simple Service Discovery Protocol address
239.255.255.253	Service Location Protocol version 2 address

IPv6

Multicast addresses in IPv6 use the prefix .

Based on the value of the flag bits, IPv6 multicast addresses can be Unicast-Prefix-based Multicast Addresses, Source-Specific Multicast Addresses, or Embedded RP IPv6 Multicast Addresses. Each of these types of multicast addresses have their own format and follow specific rules.

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

Multicast address scope!IPv6 address^[12] !IPv4 equivalent!Scope!Purpose
		Reserved
		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.
		Link-local	Packets with this destination address may not be routed anywhere.
		Realm-Local scope	Local multicast particular to a network technology
		Admin-local	The smallest scope that must be administratively configured.
		Site-local	Restricted to the local physical network.
		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.)
		Global scope	Eligible to be routed over the public internet.

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

Notable IPv6 multicast addresses

The following table is a list notable IPv6 multicast addresses that are registered with IANA.^[13] To be included in some of the below multicast groups a client must send a Multicast Listener Discovery (MLD), a component of ICMPv6 suite, to join that group. For example, to listen to, a client must send a MLD report to the router, containing the multicast address, to indicate that it wants to listen to that group.

Address	Description
	All nodes on the local network segment
	All routers on the local network segment
	OSPFv3 All SPF routers
	OSPFv3 All DR routers
	IS-IS for IPv6 routers
	RIP routers
	EIGRP routers
	PIM routers
	Virtual Router Redundancy Protocol (VRRP) version 3
	MLDv2 reports
	All DHCPv6 servers and relay agents on the local network segment
	All LLMNR hosts on the local network segment
	All DHCPv6 servers on the local network site
	Simple Service Discovery Protocol
	Multicast DNS
	Network Time Protocol
	Network Information Service
	Precision Time Protocol (PTP) version 2 messages (Sync, Announce, etc.) except peer delay measurement
	Precision Time Protocol (PTP) version 2 peer delay measurement messages
	Used for experiments

Ethernet

Ethernet frames with a value of 1 in the least-significant bit of the first octet^[14] of the destination MAC 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 are sometimes referred to as broadcasts, Ethernet 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.

The IEEE has allocated the address block to for group addresses for use by standard protocols. Of these, the MAC group addresses in the range of to are not forwarded by 802.1D-conformant MAC bridges.^[15]

--note xx-xx-xx-xx-xx-xx is preferred IEEE notation for MAC addresses. See MAC_address#Notational_conventions. Please use this format in this section.-->
+Some well known Ethernet multicast addresses^[16]	Block	Ethernet multicast address	Ethertype	Usage
01-80-C2^[17] IEEE (802 group)		Local LAN Segment, stopping at STP-capable switches
		SNAP (length)	Spanning Tree Protocol (for bridges) IEEE 802.1D
		0x88CC	Link Layer Discovery Protocol (additional)
		0x8808	Ethernet flow control (pause frame) IEEE 802.3x
		0x8809	"Slow protocols" including Ethernet OAM Protocol (IEEE 802.3ah) and Link Aggregation Control Protocol (LACP)
		Local LAN Segment until next multi-port ("non-TPMR") switch
		0x888E	Port authentication (IEEE 802.1X EAPOL)
		0x88CC	Link Layer Discovery Protocol (additional)
		SNAP (length)	Spanning Tree Protocol (for provider bridges) IEEE 802.1ad
		0x88F5	Multiple VLAN Registration Protocol (for provider bridges) IEEE 802.1ad
		Local LAN Link, never crosses another device
		0x88CC	Link Layer Discovery Protocol (primary)
		0x88F7	Precision Time Protocol (PTP) version 2 over Ethernet (802.1AS)
		0x88F5	GARP VLAN Registration Protocol (also known as IEEE 802.1Q GVRP) Multiple VLAN Registration Protocol (MVRP)
	through	0x8902
01-1B-19 IEEE (TC9)		0x88F7	Precision Time Protocol (PTP) version 2 over Ethernet (native layer-2) for electing the Grandmaster clock and advanced applications, otherwise
01-00-5E^[18] ICANN/IANA	through	0x0800	IPv4 Multicast: Insert the low 23 bits of the multicast IPv4 address into the Ethernet address
33-33-xx locally administered	through	0x86DD	IPv6 multicast: The low 32 bits an Ethernet address for IPv6 multicast traffic are the low 32 bits of the multicast IPv6 address used. For example, IPv6 multicast traffic using the address uses the MAC address, and traffic to goes to the MAC address .
01-0C-CD IEC	through	0x88B8	IEC 61850-8-1 GOOSE Type 1/1A
	through	0x88B9	GSSE (IEC 61850 8-1)
	through	0x88BA	Multicast sampled values (IEC 61850 8-1)
01-00-0C Cisco Systems		SNAP (length)	Cisco Discovery Protocol (CDP), VLAN Trunking Protocol (VTP), Unidirectional Link Detection (UDLD)
01-00-0C Cisco Systems		SNAP (length)	Cisco Shared Spanning Tree Protocol Address

802.11

802.11 wireless networks use the same MAC addresses for multicast as Ethernet.

Notes and References

https://www.ge.com/digital/documentation/cimplicity/version10/oxy_ex-2/advanced_features/topics/g_cimplicity_advanced_features_multicast_ip_addr.html MULTICAST_IP_ADDR
https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-3 AD-HOC Block 1
https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-5 SDP/SAP Block
https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-6 AD-HOC Block 2
Book: TCP/IP Illustrated . Fall, K.R. and Stevens, W.R. . 1 . 9780321336316 . 2011 . Addison-Wesley . 55.
https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-11 AD-HOC Block 3
https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#unicast-prefix-based Unicast-Prefix-based IPv4 Multicast Addresses
Book: Deploying Next Generation Multicast-Enabled Applications - Label Switched Multicast for MPLS, VPN, VPLS, and Wholesale Ethernet. Joseph. Vinod. Mugulu. Srinivas. Morgan Kaufmann/Elsevier. 2011. 978-0-12-384923-6. 7. "Lacking anything better to call it, one of the authors [of RFC2770], David Meyer, simply began refer to this as "GLOP" addressing and the name stuck.".
Web site: Frequently Asked Questions (FAQ) File for Multicasting . https://web.archive.org/web/20110516204230/http://www.multicasttech.com/faq/ . Multicast Tech . 2011-05-16.
Web site: Guidelines for Enterprise IP Multicast Address Allocation . 7 . . 2023-01-14 . most Layer 2 switches flood all multicast traffic that falls within the MAC address range of 0x0100.5E00.00xx [...] to all ports on the switch even if IGMP Snooping is enabled. [...] There are several multicast group ranges besides the 224.0.0.0/24 that will map to the 0x0100.5E00.00xx MAC address range and hence also will be flooded by most Layer 2 switches..
Web site: IPv4 Multicast Address Space Registry . 2024-06-30 . www.iana.org.
x is a place holder indicating that the value of the flags field is unimportant in the current discussion.
Web site: IPv6 Multicast Address Space Registry . Internet Assigned Numbers Authority.
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.
Web site: IEEE . Standard Group MAC Address: A Tutorial Guide . https://ghostarchive.org/archive/20221009/http://standards.ieee.org/develop/regauth/tut/macgrp.pdf . 2022-10-09 . dead . IEEE Standards Association . 2–3.
Web site: Patton, Michael A.. etal. Multicast (including Broadcast) Addresses. cavebear.com . Karl Auerbach.
Web site: Standard Group MAC Address - Public Listing . IEEE-SA . 30 November 2022.
Web site: IANA Multicast 48-bit MAC Addresses . IANA . 31 January 2021.