# 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 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 224.0.0.0/4. The group includes the addresses from 224.0.0.0 to 239.255.255.255.
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 | No |
| 224.0.1.0 to 224.0.1.255 | Internetwork control | Yes |
| 224.0.2.0 to 224.0.255.255 | AD-HOC block 1 | Yes |
| 224.1.0.0 to 224.1.255.255 | Reserved$^{:§3}$ | |
| 224.2.0.0 to 224.2.255.255 | SDP/SAP block | Yes |
| 224.3.0.0 to 224.4.255.255 | AD-HOC block 2 | Yes |
| 224.5.0.0 to 224.255.255.255 | Reserved$^{:§3}$ | |
| 225.0.0.0 to 231.255.255.255 | Reserved$^{:§3}$ | |
| 232.0.0.0 to 232.255.255.255 | Source-specific multicast | Yes |
| 233.0.0.0 to 233.251.255.255 | GLOP addressing | Yes |
| 233.252.0.0 to 233.255.255.255 | AD-HOC block 3 | Yes |
| 234.0.0.0 to 234.255.255.255 | Unicast-prefix-based | Yes |
| 235.0.0.0 to 238.255.255.255 | Reserved$^{:§3}$ | |
| 239.0.0.0 to 239.255.255.255 | Administratively scoped | Yes |

### Local subnetwork

    Addresses in the range of 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 and 224.0.0.6, and Multicast DNS uses 224.0.0.251. Routers must not forward these messages outside the subnet from 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 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 224.0.1.1.

### 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.$^{:§6}$

### SDP/SAP block

    Addresses in the reserved 224.2.0.0/16 range are not individually assigned by IANA. Fallen out of use$^{:§10}$ 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 232.0.0.0/8 (IPv4) and ff3x::/32 (IPv6) blocks are reserved for use by source-specific multicast.

### GLOP

    The 233.0.0.0/8 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. The method is not applicable to the newer 32-bit ASNs. In September 2001, 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 234.0.0.0/8 range is assigned as a range of global IPv4 multicast address space provided to each organization that has /24 or larger globally routed unicast address space allocated; one multicast address is reserved per /24 of unicast space. A resulting advantage over GLOP is that the unicast-prefix mechanism resembles the unicast-prefix capabilities of IPv6.

### Administratively scoped

    The 239.0.0.0/8 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 239.0.0.0/8 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 *.0.0.0/24 and *.128.0.0/24 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.

### 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) | No |
| 224.0.0.1 | The All Hosts multicast group addresses all hosts on the same network segment. | No |
| 224.0.0.2 | The All Routers multicast group addresses all routers on the same network segment. | No |
| 224.0.0.4 | This address is used in the Distance Vector Multicast Routing Protocol (DVMRP) to address multicast routers. | No |
| 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. | No |
| 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. | No |
| 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. | No |
| 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. | No |
| 224.0.0.13 | Protocol Independent Multicast (PIM) Version 2 | No |
| 224.0.0.18 | Virtual Router Redundancy Protocol (VRRP) | No |
| 224.0.0.19–21 | IS-IS over IP | No |
| 224.0.0.22 | Internet Group Management Protocol (IGMP) version 3$^{:§4.2.14}$ | No |
| 224.0.0.102 | Hot Standby Router Protocol version 2 (HSRPv2) / Gateway Load Balancing Protocol (GLBP) | No |
| 224.0.0.107 | Precision Time Protocol (PTP) version 2 peer delay measurement messaging | No |
| 224.0.0.251 | Multicast DNS (mDNS) address | No |
| 224.0.0.252 | Link-local Multicast Name Resolution (LLMNR) address | No |
| 224.0.0.253 | Teredo tunneling client discovery address$^{:§2.17}$ | No |
| 224.0.1.1 | Network Time Protocol clients listen on this address for protocol messages when operating in multicast mode. | Yes |
| 224.0.1.22 | Service Location Protocol version 1 general | Yes |
| 224.0.1.35 | Service Location Protocol version 1 directory agent | Yes |
| 224.0.1.39 | The Cisco multicast router AUTO-RP-ANNOUNCE address is used by RP mapping agents to listen for candidate announcements. | Yes |
| 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. | Yes |
| 224.0.1.41 | H.323 Gatekeeper discovery address | Yes |
| 224.0.1.129–132 | Precision Time Protocol (PTP) version 1 messages (Sync, Announce, etc.) except peer delay measurement | Yes |
| 224.0.1.129 | Precision Time Protocol (PTP) version 2 messages (Sync, Announce, etc.) except peer delay measurement | Yes |
| 224.0.23.12 | KNXnet/IP discovery | Yes |
| 239.255.255.250 | Simple Service Discovery Protocol address | Yes |
| 239.255.255.253 | Service Location Protocol version 2 address | Yes |

## IPv6

Multicast addresses in IPv6 use the prefix ff00::/8.
General multicast address format :
| bits | 8 | 4 | 4 | 112 |
| field | prefix | flg | sc | group ID |
For all multicast addresses, the prefix field holds the binary value 11111111.
Currently, three of the four flag bits in the flg field are defined; the most-significant flag bit is reserved for future use.
Multicast address flags :
| bit | flag | Meaning when 0 | Meaning when 1 |
| 8 | reserved | reserved | reserved |
| 9 | R (Rendezvous) | Rendezvous point not embedded | Rendezvous point embedded |
| 10 | P (Prefix) | Without prefix information | Address based on network prefix |
| 11 | T (Transient) | Well-known multicast address | Dynamically assigned multicast address |
The four-bit scope field (sc) is used to indicate where the address is valid and unique.
In addition, the scope field is used to identify special multicast addresses, like solicited node.
Solicited-node multicast address format :
| bits | 8 | 4 | 4 | 79 | 9 | 24 |
| field | prefix | flg | sc | zeroes | ones | unicast address |
The sc(ope) field holds the binary value 0010 (link-local). Solicited-node multicast addresses are computed as a function of a node's unicast or anycast addresses. A solicited-node multicast address is created by copying the last 24 bits of a unicast or anycast address to the last 24 bits of the multicast address.
Unicast-prefix-based multicast address format :
| bits | 8 | 4 | 4 | 4 | 4 | 8 | 64 | 32 |
| field | prefix | flg | sc | res | riid | plen | network prefix | group ID |
Link-scoped multicast addresses use a comparable format.
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 | IPv4 equivalent$^{:§8}$ | Scope | Purpose |
| ffx0::/16, ffxf::/16 | | Reserved |
| ffx1::/16 | | 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 | Realm-Local scope | Local multicast particular to a network technology |
| 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 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.

### Notable IPv6 multicast addresses

The following table is a list notable IPv6 multicast addresses that are registered with IANA. 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 ff02::1:ff28:9c5a, 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 |
| ff02::1 | All nodes on the local network segment |
| ff02::2 | All routers on the local network segment |
| ff02::5 | OSPFv3 All SPF routers |
| ff02::6 | OSPFv3 All DR routers |
| ff02::8 | IS-IS for IPv6 routers |
| ff02::9 | RIP routers |
| ff02::a | EIGRP routers |
| ff02::d | PIM routers |
| ff02::12 | Virtual Router Redundancy Protocol (VRRP) version 3 |
| ff02::16 | MLDv2 reports |
| ff02::1:2 | All DHCPv6 servers and relay agents on the local network segment |
| ff02::1:3 | All LLMNR hosts on the local network segment |
| ff05::1:3 | All DHCPv6 servers on the local network site |
| ff0x::c | Simple Service Discovery Protocol |
| ff0x::fb | Multicast DNS |
| ff0x::101 | Network Time Protocol |
| ff0x::108 | Network Information Service |
| ff0x::181 | Precision Time Protocol (PTP) version 2 messages (Sync, Announce, etc.) except peer delay measurement |
| ff02::6b | Precision Time Protocol (PTP) version 2 peer delay measurement messages |
| ff0x::114 | Used for experiments |

## Ethernet

Ethernet frames with a value of 1 in the least-significant bit of the first octet 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 (FF-FF-FF-FF-FF-FF) 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 01-80-C2-00-00-00 to 01-80-C2-FF-FF-FF for group addresses for use by standard protocols. Of these, the MAC group addresses in the range of 01-80-C2-00-00-00 to 01-80-C2-00-00-0F are not forwarded by 802.1D-conformant MAC bridges.

## 802.11

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