Multicast Group Concept

Multicast Group Concept

Multicast is based on the concept of a group. A multicast group is an arbitrary group of receivers that expresses an interest in receiving a particular data stream. This group has no physical or geographical boundaries—the hosts can be located anywhere on the Internet or any private internetwork. Hosts that are interested in receiving data flowing to a particular group must join the group using IGMP (IGMP is discussed in the "Internet Group Management Protocol (IGMP)" section later in this document). Hosts must be a member of the group to receive the data stream.

IP Multicast Addresses

IP multicast addresses specify a "set" of IP hosts that have joined a group and are interested in receiving multicast traffic designated for that particular group. IPv4 multicast address conventions are described in the following sections.

IP Class D Addresses

The Internet Assigned Numbers Authority (IANA) controls the assignment of IP multicast addresses. IANA has assigned the IPv4 Class D address space to be used for IP multicast. Therefore, all IP multicast group addresses fall in the range from 224.0.0.0 through 239.255.255.255.

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Note The Class D address range is used only for the group address or destination address of IP multicast traffic. The source address for multicast datagrams is always the unicast source address.

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Table 1 gives a summary of the multicast address ranges discussed in this document.

Table 1 Multicast Address Range Assignments

Description	Range
Reserved Link Local Addresses	224.0.0.0/24
Globally Scoped Addresses	224.0.1.0 to 238.255.255.255
Source Specific Multicast	232.0.0.0/8
GLOP Addresses	233.0.0.0/8
Limited Scope Addresses	239.0.0.0/8

Globally Scoped Addresses

Addresses in the range from 224.0.1.0 through 238.255.255.255 are called globally scoped addresses. These addresses are used to multicast data between organizations and across the Internet.

Some of these addresses have been reserved for use by multicast applications through IANA. For example, IP address 224.0.1.1 has been reserved for Network Time Protocol (NTP).

IP addresses reserved for IP multicast are defined in RFC 1112, Host Extensions for IP Multicasting. More information about reserved IP multicast addresses can be found at the following location:
http://www.iana.org/assignments/multicast-addresses.

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Note You can find all RFCs and Internet Engineering Task Force (IETF) drafts on the IETF website (http://www.ietf.org).

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Source Specific Multicast Addresses

Addresses in the 232.0.0.0/8 range are reserved for Source Specific Multicast (SSM). SSM is an extension of the PIM protocol that allows for an efficient data delivery mechanism in one-to-many communications. SSM is described in the "Source Specific Multicast (SSM)" section later in this document.

GLOP Addresses

RFC 2770, GLOP Addressing in 233/8, proposes that the 233.0.0.0/8 address range be reserved for statically defined addresses by organizations that already have an AS number reserved. This practice is called GLOP addressing. The AS number of the domain is embedded into the second and third octets of the 233.0.0.0/8 address range. For example, the AS 62010 is written in hexadecimal format as F23A. Separating the two octets F2 and 3A results in 242 and 58 in decimal format. These values result in a subnet of 233.242.58.0/24 that would be globally reserved for AS 62010 to use.

Limited Scope Addresses

Addresses in the 239.0.0.0/8 range are called limited scope addresses or administratively scoped addresses. These addresses are described in RFC 2365, Administratively Scoped IP Multicast, to be constrained to a local group or organization. Companies, universities, or other organizations can use limited scope addresses to have local multicast applications that will not be forwarded outside their domain. Routers typically are configured with filters to prevent multicast traffic in this address range from flowing outside of an autonomous system (AS) or any user-defined domain. Within an autonomous system or domain, the limited scope address range can be further subdivided so that local multicast boundaries can be defined. This subdivision is called address scoping and allows for address reuse between these smaller domains.