February 2000
Intermediate to advanced
464 pages
15h 57m
English
Content preview from Internet Core Protocols: The Definitive GuideBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Name
Multicast Group
Synopsis
Specifies the multicast group address that this message applies to.
- Size
Thirty-two bits.
- Default Value
The destination multicast address specified in the IP header, or “0.0.0.0”.
- Defined In
RFCs 1112 and 2236.
- Notes
Membership Queries and reports are not always sent to the multicast group address to which they pertain. Sometimes they are sent to a different address. Therefore, the contents of this field will vary, depending upon the type of message being sent.
IGMPv1 and IGMPv2 “general” queries are sent to the “all-hosts” group address of 224.0.0.1, and the Multicast Group field is set to 0.0.0.0, indicating that the query is for all groups. Group-specific queries (which are normally sent in response to a Leave Report) are also sent to the “all-hosts” address, although the Multicast Group field will be set to the IP address of the specific multicast group being queried.
Membership Reports sent by hosts on the network are sent to the IP address of the multicast group they are joining or reporting, and the Multicast Group field is set to the same multicast group address. This provides an additional validation service to the multicast routers and hosts on the network; if the destination address and the Multicast Group fields do not match, the datagram can be discarded as simply being erroneous.
Finally, IGMPv2 Leave Reports are sent to the “all-routers” group address of 224.0.0.2, while the Multicast Group field shows the IP address of the multicast group that is ...
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