Content

| 7.2 |  | Examining Multicast in a Multilayer Switched Network |
| 7.2.3 |  | Layer 3 multicast addressing |

IP multicast addresses at Layer 3 are characterized as Class D addresses. The first four bits of IP Class D addresses are 1110. This means that IP multicast addresses have a valid range from 224.0.0.0 to 239.255.255.255. Note that multicast addresses from 224.0.0.0 to 224.0.0.255 are reserved. The reserved addresses shown in Figure are specifically relevant to multicasting.

Usually, a network administrator assigns a multicast address to the application and must select an address not used by other applications or processes. The administrator should not use a multicast address in the reserved range.

Reserved Link Local Addresses
The IANA has reserved addresses from 224.0.0.0 to 224.0.0.255 to be used by network protocols on a local network segment. Packets with these addresses should never be forwarded by a router. They must remain local on a particular LAN segment and are always transmitted with a time-to-live (TTL) of 1.

Network protocols use these addresses for automatic router discovery and to communicate important routing information. For example, OSPF uses 224.0.0.5 and 224.0.0.6 to exchange link state information.

Globally Scoped Address
The address range from 224.0.1.0 through 238.255.255.255 is called Globally Scoped Address. They can be 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, 224.0.1.1 has been reserved for Network Time Protocol (NTP), used to synchronize clocks on network devices. The IANA also assigned the two group addresses 224.0.1.39 and 224.0.1.40 for Auto-RP, a PIM sparse-mode mechanism which will be discussed later in this module.

Limited Scope Addresses
The address range from 239.0.0.0 through 239.255.255.255 is known as Limited Scope Addresses or Administratively Scoped Addresses. These are defined by RFC 2365 to be constrained to a local group or organization. Routers are typically 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 will also allow for address reuse between these smaller domains.

GLOP Addressing
RFC 2770 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. The AS number of the domain is embedded into the second and third octets of the 233.0.0.0/8 range.

For example, the AS 62010 is written in hex as F23A. Separating out the two octets F2 and 3A we get 242 and 58 in decimal. This would give us a subnet of 233.242.58.0 that would be globally reserved for AS 62010 to use.