Multicast Address Structure

IP multicasting is the transmission of an IP data frame to a multicast group, identified by a single IP address. Because the multicast group is identified by a single IP address rule, the IP multicast datagram contains a specific combination of the destination MAC address and a destination IP address.

The range of IP addresses is divided into classes based on the high order bits of a 32-bit IP address. IP multicast uses Class D addresses. A Class D address consists of 1110 as the higher order bits in the first octet, followed by a 28-bit group address. Unlike Class A, B, and C IP addresses, the last 28 bits of a Class D address are unstructured, as illustrated by Figure 10-5.

Figure 10-5 IP Multicast Uses Class D Addresses

28 bits

Class D

28 bits

Class D

1

1

1

0

Multicast Group ID

These remaining 28 bits of the IP address identify the multicast group ID. This multicast group ID is a single address typically written as decimal numbers in the range 224.0.0.0 through 239.255.255.255. The high-order bits in the first octet identify this 224-base address.

Multicast addresses may be dynamically or statically allocated. Dynamic multicast addressing provides applications with a group address on demand. Because dynamic multicast addresses have a specific lifetime, applications must request this type of address only for as long as it is needed.

Statically allocated addresses are reserved for specific protocols that require well-known addresses. The Internet Assigned Numbers Authority (IANA) assigns these well-known addresses. These addresses are called permanent host groups and are similar in concept to the well-known TCP and UDP port numbers. Table 10-2 lists some of the "well-known" Class D addresses.

Address 224.0.0.1 identifies the all-hosts group. Every multicast-capable host must join this group at the start. If a ping command is issued using this address, all multicast-capable hosts on the network must answer the ping request.

Address 224.0.0.2 identifies the all-routers group. Multicast routers must join that group on all multicast-capable interfaces.

Table 10-2 Well-Known Class D Addresses

Well-Known Class D Address

Purpose

224.0.0.1

All hosts on a subnet

224.0.0.2

All routers on a subnet

224.0.0.4

All Distance Vector Multicast Routing Protocol

(DVMRP) routers

224.0.0.5

All Open Shortest Path First (OSPF) routers

224.0.0.6

All OSPF designated routers

224.0.0.9

All Routing Information Protocol, version 2 (RIP-

2) routers

224.0.0.13

All Protocol Independent Multicast (PIM) routers

Addresses ranging from 224.0.0.0 through 224.0.0.255 are reserved for local purposes, such as administrative and maintenance tasks. Multicast routers do not forward datagrams destined to this range of addresses.

Similarly, the address range 239.0.0.0 to 239.255.255.255 is reserved for administrative scoping. An administrative scope zone is defined by a set of routers surrounding a region within the network. These routers are configured to deny multicast traffic in a particular address range from entering or leaving the zone. This technique is useful in containing high-bandwidth traffic to a specific region in the campus network. Administrative scoping is outlined in RFC 2365, Administratively Scoped IP Multicast.

You can retrieve a document containing the current list of multicast assigned address at ftp://ftp.isi.edu/in-notes/iana/assignments/.

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