A multicast address is a numerical 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 address redirects here-->
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 . The group includes the addresses from to .
The address range is divided into blocks, each assigned a specific purpose or behavior.
{| class="wikitable"
|-
!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
|
|-
|224.1.0.0 to 224.1.255.255
|Reserved
|
|-
|224.3.0.0 to 224.4.255.255
|AD-HOC block 2
|
|-
|224.5.0.0 to 224.255.255.255
|Reserved
|
|-
|233.252.0.0 to 233.255.255.255
|AD-HOC block 3
|
|-
|234.0.0.0 to 234.255.255.255
|Unicast-prefix–based
|
|-
|235.0.0.0 to 238.255.255.255
|Reserved
: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. 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.
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).
|
|-
|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–rendezvous-point 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.
{| class="wikitable"
|+Multicast address scope
!IPv6 address
!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.
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.
{| class="wikitable"
! 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 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.
{| class="wikitable" <!--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
!Block
!Ethernet multicast address
!Ethertype
!Usage
|-
|rowspan="15"|
01-80-C2<br/>
<small>IEEE (802 group)</small>
|rowspan="3"|
|colspan="2" style="background-color:#def"| Local LAN Segment, stopping at STP-capable switches
|-
|SNAP (length)
|Spanning Tree Protocol (for bridges) IEEE 802.1D
|-
|0x88CC
|Link Layer Discovery Protocol (additional)
<!-------------- end -00 ---------------->
|-
|
|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)
|-
|rowspan="3"|
|colspan="2" style="background-color:#def"| 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
|-
|rowspan="3"|
|colspan="2" style="background-color:#def"| 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)<br/>
Multiple VLAN Registration Protocol (MVRP)
|-
| through<br/>
|0x8902
|Ethernet CFM Protocol IEEE 802.1ag
|-
|rowspan="1"|
01-1B-19<br/>
<small>IEEE (TC9)</small>
|
|0x88F7
|Precision Time Protocol (PTP) version 2 over Ethernet (native layer-2)<br/>for electing the Grandmaster clock and advanced applications, otherwise
|-
|rowspan="1"|
01-00-5E<br/>
<small>ICANN/IANA</small>
| through<br/>
|0x0800
|IPv4 Multicast: Insert the low 23 bits of the multicast IPv4 address into the Ethernet address
|-
|rowspan="1"|
33-33-xx<br/>
<small>locally administered</small>
| through<br/>
|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 .
|-
|rowspan="3"|
01-0C-CD<br/>
<small>IEC</small>
| through<br/>
|0x88B8
|IEC 61850-8-1 GOOSE Type 1/1A
|-
| through<br/>
|0x88B9
|GSSE (IEC 61850 8-1)
|-
| through<br/>
|0x88BA
|Multicast sampled values (IEC 61850 8-1)
|-
|rowspan="2"|
01-00-0C<br/>
<small>Cisco Systems</small>
|
|SNAP (length)
|Cisco Discovery Protocol (CDP), VLAN Trunking Protocol (VTP), Unidirectional Link Detection (UDLD)
|-
|
|SNAP (length)
|Cisco Shared Spanning Tree Protocol Address
|}
802.11
802.11 wireless networks use the same MAC addresses for multicast as Ethernet.
See also
- Broadcast address
- Reserved IP addresses
Notes
References
</references>