In my previous, I tried to reason why IP multicast is required and how does it help in the network. In this post, I will focus on multicast addressing.
IP multicast uses Class D address space. First address is 224.0.0.0 and last address is 239.255.255.255. This range is also referred as 224/4.
Similar to unicast address space, some of the multicast address ranges are special. 224.0.0.0/24 are known as local multicast addresses. Packets destined to these addresses are not forwarded by a router and treated as broadcast packets by switches. Addresses in this range is used by various protocols like RIP, OSPF etc. to communicate with immediate neighbors. 232/8 are used only for Source Specific Multicast. I will talk about different types and how they are used in greater details in some future post.
IP multicast is built upon the assumption that senders and receivers do not need to know where the other is sitting in a network. They just need to know the multicast address that they need to use. Receivers shows its interest by sending out an IGMP/MLD join request with the multicast address they are interested in while senders just sends multicast data using the multicast address.
So how does an application finds out which IP multicast address to use? Very simple. Pick any multicast address and start sending data using that address. Am I kidding? No. This is how it is suppose to be. But then how do receivers suppose to know which multicast address sender is using? Some out-of-band mechanism usually manual configuration.
But should not there be some means for sender to decide on a multicast address so that it is not in conflict with others. To start with, IP Multicast did not had any allocation mechanism. People came up with various mechanism to allocate multicast addresses like MALLOC but nothing was widely deployed. Later, some mechanisms like GLOP addressing, SSM address space, Administrative scope address range etc. were defined to help administrator with a range of multicast group address they can use within their network or globally. So usually sender is configured with a multicast address that it should use.
Now a days, Applications uses an address from administrative scoped address range if multicast traffic is not suppose to go out of its Autonomous System (AS). Application can use an address from the GLOP multicast address range or SSM address range as it will be unique globally. Receivers are told about this address through out-of-band mechanisms like passing it back in a http response.
If you want a consolidated information on Multicast address architecture, please read through RFC 6308. A whole list of the special multicast addresses can be found in Wikipedia. IANA maintains the IP multicast address space registry here.
IP multicast uses Class D address space. First address is 224.0.0.0 and last address is 239.255.255.255. This range is also referred as 224/4.
Similar to unicast address space, some of the multicast address ranges are special. 224.0.0.0/24 are known as local multicast addresses. Packets destined to these addresses are not forwarded by a router and treated as broadcast packets by switches. Addresses in this range is used by various protocols like RIP, OSPF etc. to communicate with immediate neighbors. 232/8 are used only for Source Specific Multicast. I will talk about different types and how they are used in greater details in some future post.
IP multicast is built upon the assumption that senders and receivers do not need to know where the other is sitting in a network. They just need to know the multicast address that they need to use. Receivers shows its interest by sending out an IGMP/MLD join request with the multicast address they are interested in while senders just sends multicast data using the multicast address.
So how does an application finds out which IP multicast address to use? Very simple. Pick any multicast address and start sending data using that address. Am I kidding? No. This is how it is suppose to be. But then how do receivers suppose to know which multicast address sender is using? Some out-of-band mechanism usually manual configuration.
But should not there be some means for sender to decide on a multicast address so that it is not in conflict with others. To start with, IP Multicast did not had any allocation mechanism. People came up with various mechanism to allocate multicast addresses like MALLOC but nothing was widely deployed. Later, some mechanisms like GLOP addressing, SSM address space, Administrative scope address range etc. were defined to help administrator with a range of multicast group address they can use within their network or globally. So usually sender is configured with a multicast address that it should use.
Now a days, Applications uses an address from administrative scoped address range if multicast traffic is not suppose to go out of its Autonomous System (AS). Application can use an address from the GLOP multicast address range or SSM address range as it will be unique globally. Receivers are told about this address through out-of-band mechanisms like passing it back in a http response.
If you want a consolidated information on Multicast address architecture, please read through RFC 6308. A whole list of the special multicast addresses can be found in Wikipedia. IANA maintains the IP multicast address space registry here.
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