by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568 Pub Date: 11/01/98
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Illustrated TCP/IP
by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568 Pub Date: 11/01/98 |
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Multicasting for IP started in 1988 with IGMP. IANA also assigned a new class of addressing known as Class D addressing. Multicasting is carried over to IPv6 and its addressing allows for more granularity. Multicasting is used extensively with IPv6. The format of the address is shown in the slide. The first 8 bits must be set to FF. The next 4 bits are called the flag bits, of which only one is defined. The T bit is the transient bit. Setting this to 1 indicates the multicast address is not permanently assigned by the IANA. A 0 indicates it is permanently assigned.
The scope is 4 bits in length and controls the “hearing range” of the multicast address. It performs the same function as the TTL field in an IPv4 multicast packet. The following table indicates what scopes are currently assigned.
| Scope | Range |
|---|---|
| 0 | Reserved |
| 1 | Node local scope |
| 2 | Link local scope |
| 3 | Unassigned |
| 4 | Unassigned |
| 5 | Site local scope |
| 6 | Unassigned |
| 7 | Unassigned |
| 8 | Organization local scope |
| 9 | Unassigned |
| A | Unassigned |
| B | Unassigned |
| C | Unassigned |
| D | Unassigned |
| E | Global scope |
| F | Reserved |
Notice that in IPv6 multicast addresses, weaving the scope in as part of the address makes it possible to have multiple multicast addresses for the same function. The first part of the address is the multicast address identifier, but the scope is included in the overall address. This allows for multiple multicast addresses to be assigned to the same function. For example, there is one multicast address looking for all DHCP servers in a radius of 3 hops. Another would allow for a radius of 10 hops, but it is still the same multicast function.
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