Multicast

I had always mixed up between broadcast and multicast but not anymore, they do have similarity i.e. both send each copy of the packet to every nodes. But there’s a significant difference.

Broadcast is like a brute, a node sends a broadcast; all nodes within the subnet must “listen”, there’s no choice for node to leave the broadcast group, even if the broadcast is not meant for some nodes the nodes that received the broadcast have to swallow it.

Multicast is more graceful, multicast forwards packets to nodes that belong to a group which is identify by Multicast group id. Only nodes belong to the MC address group receive the MC packets, those out of the group nodes will never receive it.

Multicast belongs to class D network from 224.0.0.0 to 239.255.255.255

The MSB is 1110 the remaining 28-bit belongs to multicast group id.

There are reserved multicast addresses, application cannot use any MC address that it likes.

Reserved link local MC address:

224/8 – popular ones

1. OSPF (224.0.0.5 and 224.0.0.6), only OSPF nodes will receive the MC, other nodes within the network with no OSPF route enabled will not need to swallow the MC.

2. EIGRP (224.0.0.10), only nodes that have EIGRP enabled will receive the MC information, other nodes without EIGRP will not receive it.

3. HSRP version 2 (224.0.0.102), HSRP version 2 enabled routers will receive it, HSRP version 1 and other non-HSRPv2 will not receive it.

Globally scoped address:

224.0.1.0 – 238.255.255.255

All the addresses except 224.0.1.1 (NTP MC) are free to use by application to transmit packets to MC grouped nodes via organisation or via internet.

Source Specific MC

232/8

Perhaps use by MC routing like PIM and DVMRP. Routing is based on source MC address and group MC address.

GLOP MC

233/8

Do not understand the use.

Limited scope MC

239/8

Similar to RFC1918 type of private addressing, limited to local network within an organisation, this range is not used for data transmission via internet only for local network (intranet)