Well Known Multicast Addresses

IANA controls the assignment of IP multicast addresses. To preserve multicast addresses, IANA is reluctant to assign individual IP multicast addresses to new applications without a good technical justification. However, IANA has assigned individual IP multicast addresses to popular network protocols.

IANA has assigned several ranges of multicast IP addresses for specific types of reasons. Those types are as follows:

KEY ■ Permanent multicast groups, in the range 224.0.0.0-224.0.1.255 POINT

■ Addresses used with Source-Specific Multicast (SSM), in the range 232.0.0.0-232.255.255.255

■ GLOP addressing, in the range 233.0.0.0-233.255.255.255

■ Private multicast addresses, in the range 239.0.0.0-239.255.255.255

This section provides some insights into each of these four types of reserved IP multicast addresses. The rest of the multicast addresses are referred to as transient groups, which are covered later in this chapter in the section "Multicast Addresses for Transient Groups."

Multicast Addresses for Permanent Groups

IANA has reserved two ranges of permanent multicast IP addresses. The main distinction between these two ranges of addresses is that the first range is used for packets that should not be forwarded by routers, and the second group is used when packets should be forwarded by routers.

The range of addresses used for local (not routed) purposes is 224.0.0.0 through 224.0.0.255. These addresses should be somewhat familiar from the routing protocol discussions earlier in the book; for example, the 224.0.0.5 and 224.0.0.6 IP addresses used by OSPF fit into this first range of permanent addresses. Other examples include the IP multicast destination address of 224.0.0.1, which specifies that all multicast-capable hosts on a local network segment should examine this packet. Similarly, the IP multicast destination address of 224.0.0.2 on a packet specifies that all multicast-capable routers on a local network segment should examine this packet.

The range of permanent group addresses used when the packets should be routed is 224.0.1.0 through 224.0.1.255. This range includes 224.0.1.39 and 224.0.1.40, which are used by Cisco-proprietary Auto-Rendezvous Point (Auto-RP) protocols (covered in Chapter 20). Table 19-2 shows some of the well-known addresses from the permanent address range.

Table 19-2 Some Well-Known Reserved Multicast Addresses

KEY POINT

Table 19-2 Some Well-Known Reserved Multicast Addresses

KEY POINT

Address

Usage

224.0.0.1

All multicast hosts

224.0.0.2

All multicast routers

224.0.0.4

DVMRP routers

224.0.0.5

All OSPF routers

224.0.0.6

OSPF designated routers

224.0.0.9

RIPv2 routers

224.0.0.10

EIGRP routers

224.0.0.13

PIM routers

224.0.0.22

IGMPv3

224.0.0.25

RGMP

224.0.1.39

Cisco-RP-Announce

224.0.1.40

Cisco-RP-Discovery

Multicast Addresses for Source-Specific Multicast Applications and Protocols

IANA has allocated the range 232.0.0.0 through 232.255.255.255 for SSM applications and protocols. The purpose of these applications is to allow a host to select a source for the multicast group. SSM makes multicast routing efficient, allows a host to select a better-quality source, and helps network administrators minimize multicast denial-of-service (DoS) attacks.

NOTE Only IGMPv3-capable hosts can use the SSM feature. IGMPv3 is a new protocol. At the time of this writing, a very limited number of IGMPv3 applications were available. Hence, use of these addresses is minimal.

Multicast Addresses for GLOP Addressing

IANA has reserved the range 233.0.0.0 through 233.255.255.255 (RFC 2770), called GLOP addressing, on an experimental basis. It can be used by anyone who owns a registered autonomous system number (ASN) to create 256 global multicast addresses that can be owned and used by the entity. IANA reserves addresses to ensure global uniqueness of addresses; for similar reasons, each autonomous system should be using an assigned unique ASN.

By using a value of 233 for the first octet, and by using the ASN for the second and third octets, a single autonomous system can create globally unique multicast addresses as defined in the GLOP addressing RFC. For example, the autonomous system using registered ASN 5663 could covert ASN 5663 to binary (0001011000011111). The first 8 bits, 00010110, equals 22 in decimal notation, and the last 8 bits, 00011111, equals 31 in decimal notation. Mapping the first 8 bits to the second octet and the last 8 bits to the third octet in the 233 range addresses, the entity who owns the ASN 5663 is automatically allocated the address range 233.22.31.0 through 233.22.31.255.

NOTE GLOP is not an acronym and does not stand for anything. One of the authors of RFC 2770, David Meyer, started referring to this range of addresses as "GLOP" addressing, and since then the range has been identified by the name GLOP addressing.

Multicast Addresses for Private Multicast Domains

The last of the reserved multicast address ranges mentioned here is the range of administratively scoped addresses. IANA has assigned the range 239.0.0.0 through 239.255.255.255 (RFC 2365) for use in private multicast domains, much like the IP unicast ranges defined in RFC 1918, namely 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. IANA will not assign these administratively scoped multicast addresses to any other protocol or application. Network administrators are free to use multicast addresses in this range; however, they must configure their multicast routers to ensure that multicast traffic in this address range does not leave their multicast domain boundaries.

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    Are well known multicast groups joined by default?
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