IP Multicast - Java Network Programming - page 448

Like any IP address, a multicast address can have a hostname. For example, the multicast
address 224.0.1.1 (the address of the Network Time Protocol distributed service) is
assigned the name ntp.mcast.net .
A multicast group is a set of Internet hosts that share a multicast address. Any data sent
to the multicast address is relayed to all the members of the group. Membership in a
multicast group is open; hosts can enter or leave the group at any time. Groups can be
either permanent or transient. Permanent groups have assigned addresses that remain
constant, whether or not there are any members in the group. However, most multicast
groups are transient and exist only as long as they have members. All you have to do to
create a new multicast group is pick a random address from 225.0.0.0 to 238.255.255.255,
construct an InetAddress object for that address, and start sending it data.

The IANA is responsible for handing out permanent multicast addresses as needed. So
far, a few hundred have been specifically assigned. Link-local multicast addresses begin
with 224.0.0 (i.e., addresses from 224.0.0.0 to 224.0.0.255) and are reserved for routing
protocols and other low-level activities, such as gateway discovery and group member‐
ship reporting. For example, all-systems.mcast.net , 224.0.0.1, is a multicast group that
includes all systems on the local subnet. Multicast routers never forward datagrams with
destinations in this range. Table 13-1 lists a few of these assigned addresses.

Table 13-1. Link-local multicast addresses

Domain name IP address Purpose
BASE-ADDRESS.MCAST.NET 224.0.0.0 The reserved base address. This is never assigned to any multicast
group.
ALL-SYSTEMS.MCAST.NET 224.0.0.1 All systems on the local subnet.
ALL-ROUTERS.MCAST.NET 224.0.0.2 All routers on the local subnet.
DVMRP.MCAST.NET 224.0.0.4 All Distance Vector Multicast Routing Protocol (DVMRP) routers on this
subnet.
MOBILE-AGENTS.MCAST.NET 224.0.0.11 Mobile agents on the local subnet.
DHCP-AGENTS.MCAST.NET 224.0.0.12 This multicast group allows a client to locate a Dynamic Host
Configuration Protocol (DHCP) server or relay agent on the local subnet.

RSVP-
ENCAPSULATION.MCAST.NET

224.0.0.14 RSVP encapsulation on this subnet. RSVP stands for Resource
reSerVation setup Protocol, an effort to allow people to reserve a
guaranteed amount of Internet bandwidth in advance for an event.
VRRP.MCAST.NET 224.0.0.18 Virtual Router Redundancy Protocol (VRRP) Routers
224.0.0.35 DXCluster is used to announce foreign amateur (DX) stations.
224.0.0.36 Digital Transmission Content Protection (DTCP), a digital restrictions
management (DRM) technology that encrypts interconnections
between DVD players, televisions, and similar devices.
224.0.0.37 - 224.0.0.68 zeroconf addressing
224.0.0.106 Multicast Router Discovery