multicast address

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

IP Multicast Addresses

Within IP 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 224.0.0.0/4. Address assignments out of 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 to multicasting on the local LAN only. Well known examples are RIP which uses 224.0.0.9 and OSPF which uses 224.0.0.5. 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).

P multicast address	Description
24.0.0.0	Base address (reserved)
24.0.0.1	The All Hosts multicast group that contains all systems on the same network segment
24.0.0.2	The All Routers multicast group that contains all routers on the same network segment
24.0.0.5	The Open Shortest Path First (OSPF) AllSPFRouters address. Used to send OSPF routing information to all OSPF routers on a network segment
24.0.0.6	The OSPF AllDRouters address. Used to send OSPF routing information to OSPF designated routers on a network segment
24.0.0.9	The RIP Version 2 group address. Used to send RIP routing information to all RIP v2 routers on a network segment

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

Ethernet Multicast Addresses

Some well known Ethernet multicast addresses:

thernet multicast address	Type Field	Usage
1-00-0C-CC-CC-CC	0x0802	CDP (Cisco Discovery Protocol), VTP (Virtual Trunking Protocol)
1-00-0C-CC-CC-CD	0x0802	SSTP address
1-80-C2-00-00-00	0x0802	Spanning Tree Protocol (for bridges) 802.1D
3-00-00-20-00-00	0x0802	IP Multicast Address (RFC1469)
3-33-00-00-00-00	0x86DD	IPv6 Neighbor Discovery

Complete list: http://www.cavebear.com/CaveBear/Ethernet/multicast.html

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