Multicast can be used to send IP packets to a group of interested receivers. Compared with broadcast, the packets are only sent to the members of a multicast group with the help of a multicast router.


Multicast Addresses


Class D addresses ( – are assigned to multicast. So the first 4 bits of a multicast address should be 1110 and the remaining 28 bits represent different groups. Some addresses are reserved and have been assigned to specified groups. For example, – all hosts group, all hosts configured IP multicast will join this group when an interface starts up. – all multicast routers group – IGMP Multicast router. An IGMP member report will be sent this addresses when a host joins a multicast group, then the multicast router can receive that report and maintain the membership of a group. We can capture the following packet when a program joins the multicast group


Enabling multicast on an IP address


1. To check if multicast is enabled already on an IP address using the “ifconfig” command.

# ifconfig eth0 | grep -i multi


Note: Please replace ethX with appropriate Ethernet. If multicast is enabled, a flag will be displaying.


2. To disable multicast on a specific ip/interface:

# ifconfig ethX -multicast


3. To enable multicast on a specific ip/interface :

# ifconfig ethX multicast


Listing IPs subscribed to a multicast group


In order to look at the IP address which has subscribed the multicast group, one can take a look at in the content of the following file /proc/net/igmp. For example:

# cat /proc/net/igmp
Idx	Device    : Count Querier	Group    Users Timer	Reporter
1	lo        :     1      V3
				010000E0     1 0:00000000		0
2	eth0      :     1      V3
				010000E0     1 0:00000000		0


Based on multicast application, IP will be receiving or transferring packets.


Was this answer helpful? 0 Users Found This Useful (0 Votes)