Basics of Multicast Addresses ( 224.0.0.0 to 239.255.255.255)

Today I am going to talk about basics of Multicast from the beginning. Before starting the concept of multicast you need to know about the unicast and the broadcast type of networks.

Unicast: Unicast is the traffic from from the single user to the single destination.
Broadcast: Broadcast is the traffic flow from the single user to everyone in the network.
Multicast: Multicast is the traffic flow from the single user to the specific group of the destinations in the network.

Fig 1.1- Multicast Vs Unicast

IP Multicast

IP multicast works as follows :

• Multicast groups are identified by IP addresses in the range 224.0.0.0 - 239.255.255.255 (class D address) 
• Every host (more precisely: interface) can join and leave a multicast group dynamically : no access control 
• Every IP datagram sent to a multicast group is transmitted to all members of the group
• IP multicast service is unreachable
• IP multicast only supports UDP as higher Layer which means there is no multicast TCP.
• All D class addresses are Multicast addresses. The addresses range are shown above from 224.0.0.0 to 239.255.255.255
• These addresses are dynamically assigned
• An IP datagram sent to multicast address is forward to everyone who has joined the multicast group.
• If an application is terminated, the multicast address is released

Types of Multicast Addresses

• The range of addresses between 224.0.0.0 and 224.0.0.255, inclusive, is reserved for the use of routing protocols and other low-level topology discovery or maintenance protocols 
• Multicast routers should not forward any multicast datagram with destination addresses in this range. 

Example for Special and reserved Class D addresses

• 224.0.0.1 : All systems on the subnet
• 224.0.0.2 : All routers on the subnet
• 224.0.1.1 : NTP ( Network Time Protocol)
• 224.0.0.9 : RIP Protocol

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