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, such as Ethernet multicast, and at
the Internet Layer for Internet Protocol Version 4 or Version 6 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 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 Best Current Practice document.
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.
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 uses 224.0.0.9, Open Shortest Path First uses
224.0.0.5 and 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.
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.
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.
Source-specific multicast The 232.0.0.0/8 and FF3x::/32 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 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.
Unfortunately, with only 256 multicast addresses available to each autonomous system, GLOP
is not adequate for large-scale broadcasters.
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; one multicast address is
reserved per /24 of unicast space.
A resulting advantage over GLOP is that the mechanisms in IPv4 and IPv6 become more similar.
Administratively Scoped IPv4 Multicast addresses The 239.0.0.0/8 range is assigned by RFC 2365
for private use within an organization.
From the RFC, 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 RFC also discusses structuring the 239.0.0.0/8 range to be loosely similar to the scoped
IPv6 multicast address range described in RFC 1884.
IPv6 Multicast addresses in IPv6 have the prefix
ff00::/8.
IPv6 multicast addresses are generally formed from four bit groups, illustrated as follows:
The prefix holds the binary value 11111111 for any multicast address.
Currently, 3 of the 4 flag bits in the flags field are defined; the most-significant flag
bit is reserved for future use.
The other three flags are known as R, P and T.
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 field is used to indicate where the address is valid and unique.
The service is identified in the 112-bit Group ID field.
For example, if ff02::101 refers to all Network Time Protocol 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.
Ethernet Ethernet frames with a value of 1 in the least-significant
bit of the first octet 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 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.
802.11 802.11 wireless networks use the same 01:00:5E:xx:xx:xx
and 33:33:xx:xx:xx:xx MAC addresses for multicasting as Ethernet.
See also Broadcast address
Notes
References