Multicast address

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In computer networking a multicast address is an identifier for a group of hosts that have joined a multicast group.

1 IPv4 multicast addresses
2 IPv6 multicast addresses
3 Ethernet multicast addresses
4 See also

[edit] IPv4 multicast addresses

Within IPv4 it is historically known as a class D address, a type of IP address. It ranges from 224.0.0.0 to 239.255.255.255, or, equivalently, 224.0.0.0/4 (see Classless Inter-Domain Routing). Address assignments from within this range are specified in the IETF 'best current practice' document also known as RFC 3171.

The 224.0.0.0 to 224.0.0.255 range is assigned for multicasting on the local LAN only. Well known examples are RIPv2 which uses 224.0.0.9 and OSPF which uses the 224.0.0.5 address.

The following table is a partial list of well-known Class D addresses that are reserved for IP multicasting and registered with the Internet Assigned Numbers Authority (IANA).

IP multicast address	Description
224.0.0.0	Base address (reserved)
224.0.0.1	The All Hosts multicast group that contains all systems on the same network segment
224.0.0.2	The All Routers multicast group that contains all routers on the same network segment
224.0.0.5	The Open Shortest Path First (OSPF) AllSPFRouters address. Used to send Hello packets to all OSPF routers on a network segment
224.0.0.6	The OSPF AllDRouters address. Used to send OSPF routing information to OSPF designated routers on a network segment
224.0.0.9	The RIP Version 2 group address. Used to send RIP routing information to all RIP v2 routers on a network segment
224.0.0.10	EIGRP group address. Used to send EIGRP routing information to all EIGRP routers on a network segment
224.0.0.18	Virtual Router Redundancy Protocol
224.0.0.22	IGMP (Internet Group Management Protocol)
224.0.0.102	Hot Standby Router Protocol Version 2
224.0.1.41	H.323 Gatekeeper discovery address

For a full and current listing of additional IP addresses that are reserved for multicasting, see the Internet Multicast Addresses Web site.

The 233.0.0.0/8 range has been assigned by RFC 2770 as public multicast address space that is aligned along autonomous system number allocations. Each byte of the AS number corresponds to the second and third octets of the multicast address.

The 239.0.0.0/8 range has been assigned by RFC 2365 as a locally administered address space with local or organizational scope.

For advice on choosing multicast addresses for use in your application, see this page on multicast address assignment.

[edit] IPv6 multicast addresses

Multicast addresses in IPv6 all have the prefix ff00::/8. Address assignments from within this range are specified in RFC 2373.

Bits 8 to 11 of the address (i.e., the third hexadecimal digit) are a set of flags, of which only the fourth is currently defined: 0 indicates an address permanently assigned by the IANA, and 1 indicates a non-permanently assigned or "transient" address. (In the addresses given below, an "x" indicates that the value of the digit in that position is unimportant in the current discussion.)

Similar to unicast addresses, IPv6 multicast addresses have different scopes depending on their prefix, though the range of possible scopes is different. The scope is indicated by bits 12 to 15:

ffx0::/16 and ff0f::/16 are reserved.
ffx1::/16 is interface-local, meaning 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 is link-local, meaning packets with this destination address may not be routed anywhere.
ffx4::/16 is admin-local, i.e. "the smallest scope that must be administratively configured".
ffx5::/16 is site-local, i.e. restricted to the local physical network.
ffx8::/16 is organization-local, i.e. 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 is global scope, i.e. eligible to be routed over the public internet.
Other scopes are unassigned.

These scopes are described in RFC 4291.

The "meaning" of a multicast address is unaffected by its scope; for example, if ff02::43 refers to all NTP servers on the local network segment, then ff08::43 refers to all NTP servers in an organization's networks.

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

Address	Description
ff02::1	All nodes on the local network segment (equivalent to the IPv4 link-local broadcast address, 169.254.255.255)
ff02::2	All routers on the local network segment
ff05::1	All nodes on the local network site (roughly equivalent to the IPv4 local broadcast addresses 10.255.255.255 and 192.168.255.255)
ff0x::fb	Multicast DNS
ff0x::108	Network Information Service
ff0x::114	Used for experiments
ff05::1:3	All DHCP servers on the local network site (defined in RFC 3315)

For a full and current listing of additional addresses that are allocated for multicasting, see the Internet Protocol Version 6 Multicast Addresses web site.

[edit] Ethernet multicast addresses

Some well known Ethernet multicast addresses:

Ethernet multicast address	Type Field	Usage
01-00-0C-CC-CC-CC	0x0802	CDP (Cisco Discovery Protocol), VTP (Virtual 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
01-00-5E-xx-xx-xx	0x0800	IPv4 IGMP Multicast Address
33-33-00-00-00-00	0x86DD	IPv6 Neighbor Discovery
33-33-xx-xx-xx-xx	0x86DD	IPv6 Multicast Address (RFC3307)