Multicast addresses are the IP addresses that refer to a group of hosts present in the same or on different networks. We refer to this group of hosts as a multicast group. Multicast addresses are ways to distribute a common data stream or service to the hosts in multicast addresses.
Multicast Addresses in IPv4
Addressing in IPv4 can be classified into two kinds of addressing i.e., classful addressing and classless addressing. In our earlier content, we have studied both classful addressing and classless addressing. In classful addressing the block, D is assigned for multicast addressing and the same block of address is assigned to multicast addresses in classes addressing. This block has addresses from 220.127.116.11 to 239,255.255.255.
This multicast address space is further divided into some smaller ranges or smaller blocks, based on RFC 3171. These smaller blocks can be further divided into sub-blocks.
1. Local Network Control Block
This address block ranges from 18.104.22.168 to 22.214.171.124 and is used by the protocol that is used for controlling traffic on the network. Packets with this destination address cannot be forwarded by the router and it remains in the same network. So, two or more than two networks can use the same address belonging to this block at the same time.
2. Internetwork Control Block
This address block ranges from 126.96.36.199 to 188.8.131.52 and is used by the protocol that is used for controlling the traffic on the network. Still, the packets having destination addresses from this block can be forwarded by the router throughout the internet.
3. AD-HOC Block
This address block ranges from 184.108.40.206 to 220.127.116.11. This address is assigned to the application that does not fit in the first and second blocks as we discussed above.
4. Stream Multicast Group Block
This address block 18.104.22.168/16 ranges from 22.214.171.124 to 126.96.36.199 and is referred to as Stream Multicast Group Block. The addresses of this block are used for streaming multimedia such as video and audio.
5. SAP/SDP Block
This address block 188.8.131.52/16 ranges from 184.108.40.206 to 220.127.116.11 and is used for Session Announcement Protocol and Session Directory Protocol (RFC 2974). It used to advertise the information about the multicast sessions.
6. SSM Block
This address block 18.104.22.168/8 ranges from22.214.171.124 to 126.96.36.199 and is used for source-specific multicast (SSM). Source-specific multicast is a technique of delivering the multicast packet to the receiver. Here the receiver has to specify the source from which it wants to receive the multicast packet.
7. GLOP Block
This block reserves the addresses from 188.8.131.52 to 184.108.40.206. This range has globally assigned addresses that are used inside an autonomous system.
8. Administratively Scoped Block
This block of addresses ranges from 220.127.116.11 to 18.104.22.168. Addresses belonging to this block are used in a specific area of the internet. Packets having these addresses in their destination field cannot leave that particular area. This block of addresses is usually assigned to the organization.
Selecting Multicast Address
Assigning a multicast address to a group of hosts depends on the type of application used.
1. Limited Group
If the hosts of a multicast group are inside an autonomous system, then the multicast group can choose the address from the administratively scoped block. Two multicast groups cannot have the same address from the administratively scoped blocked. Packets with multicast addresses from the administratively scoped blocks cannot go beyond an autonomous system that it belongs to.
2. Large Group
If the hosts of a multicast group are scattered beyond the autonomous system, then this multicast group needs to select an address from the SSM block as we discussed above. No permission is required to use addresses from this block.
The packets with the destination address from the SSM block are routed on the basis of the multicast group and the specified source.
Delivery of Multicast Packet at Data Link Layer
1. Network with Multicast Support
Most LANs such as Ethernet supports multicasting. Now in Ethernet, the physical address of a host is represented by 48 bits. Among these 48 bits, the first 25 bits determine the physical multicast address and the remaining 23 bits determine the multicast group to which the host belongs.
2. Network with no Multicast Support
Usually, the WANs do not support physical multicast addressing it uses a tunneling process to the pass-through these networks. Tunneling makes the multicast packet get encapsulated in the unicast packet to pass through the network.
So, this is all about the multicast addresses which is used to deliver the packet to multiple hosts in the same or different network.