Illustrated TCP/IP 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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Chapter 257
Address Format

We know all about the address formats of Classes A, B, and C. The address format that allows for multicast address is known as the Class D address. It is reserved by IANA and has a range of 224.0.0.0 to 239.255.255.255. This allows for 228 bits (groups) for multicast addressing. The base address, 224.0.0.0, is reserved and cannot be assigned to any group. Furthermore, IANA has reserved the range of 224.0.0.1 through 224.0.0.255 for the use of routing protocols, topology discovery, and maintenance protocols. What is interesting is that no router that receives a datagram with this address range is allowed to forward it. It must either consume or discard (filter) the datagram. Other addresses of interest according to RFC 1700 are:

224.0.0.0 Base Address (Reserved) [RFC1112]
224.0.0.1 All Systems on This Subnet [RFC1112]
224.0.0.2 All Routers on This Subnet
224.0.0.3 Unassigned
224.0.0.4 DVMRP Routers [RFC1075]
224.0.0.5 OSPFIGP OSPFIGP All Routers [RFC1583]
224.0.0.6 OSPFIGP OSPFIGP Designated Routers [RFC1583]
224.0.0.7 ST Routers [RFC1190]
224.0.0.8 ST Hosts [RFC1190]
224.0.0.9 RIP2 Routers [RFC1723]
224.0.0.10 IGRP Routers [Cisco]
224.0.0.11 Mobile-Agents
224.0.0.12 DHCP Server / Relay Agent [RFC1884]
224.0.0.12–224.0.0.255 Unassigned [IANA]

Address Format

•  IP multicast addresses are Class D addresses.
•  They range from 224.0.0.0 through 239.255.255.255.
•  The address of 224.0.0.0 is reserved.
•  The address range of 224.0.0.1 through 224.0.0.255 is reserved for the use of routing protocols.
•  Not necessarily generic multicast datagrams. OSPF uses 224.0.0.5 as an AllOSPFRouters multicast address.
•  The reserved ranges are not allowed to be forwarded by a router.
•  It must either consume or discard the datagram.


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