I was going through the use cases of GRE tunnel and see that it is also used to carry multicast traffic over the internet.I understand that multicast IP belongs to
class D address which is a public IP and can be routed.
Q1:Why is multicast not supported over the internet ? Why do we need to encapsulate in GRE ? Can you please explain in laymen terms ?
Q2:When we use the term 'internet', what exactly does it refer to? Would it be collection of all ISPs across the world?