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4.4.4 Multicast
Traditionally, IP packets are transmitted in one of either two ways - Unicast (1
sender - 1 recipient) or Broadcast (1 sender - everybody on the network). Multicast
delivers IP packets to a group of hosts on the network - not everybody and not just
1.
IGMP (Internet Group Multicast Protocol) is a network-layer protocol used to
establish membership in a Multicast group - it is not used to carry user data. IGMP
version 2 (RFC 2236) is an improvement over version 1 (RFC 1112) but IGMP
version 1 is still in wide use. If you would like to read more detailed information
about interoperability between IGMP version 2 and version 1, please see sections 4
and 5 of RFC 2236. The class D IP address is used to identify host groups and can
be in the range 224.0.0.0 to 239.255.255.255. The address 224.0.0.0 is not
assigned to any group and is used by IP multicast computers. The address
224.0.0.1 is used for query messages and is assigned to the permanent group of all
IP hosts (including gateways). All hosts must join the 224.0.0.1 group in order to
participate in IGMP. The address 224.0.0.2 is assigned to the multicast routers
group.
The ADSL Router supports both IGMP version 1 (IGMP-v1) and IGMP version 2
(IGMP-v2). At start up, the ADSL Router queries all directly connected networks to
gather group membership. After that, the ADSL Router periodically updates this
information. IP multicasting can be enabled/disabled on the ADSL Router LAN
and/or WAN interfaces in the web configurator (LAN; WAN). Select None to disable
IP multicasting on these interfaces.
