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If a wireless modem router has the option Allow multicast from Broadband Network, what does this mean exactly?

What risks are there to leaving it enabled and why would you have it enabled?

I understand that a broadcast is sending a packet to every device on the network and a multicast is send a packet to a specified set of addresses. A multicast packet can cross routers whereas a broadcast cannot although I don't quite understand how these "set of addresses are specified"

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up vote 4 down vote accepted

IP multicast is a method by which data can be broadcasted to a select group of hosts using a single transmission. These days, it's primarily used for IPTV and satellite connections. In theory, it could be used for any kind of streaming broadcast-type media (e.g. internet TV, internet radio, teleconferencing, and other Video-over-IP applications), and the more hosts on a network that subscribe to a multicast stream, the more efficient it becomes.

However, the only consumer-level implementation of it that I know of is IPTV. In this case, the user's set-top box is the multicast/IGMP client, and their home router would be the multicast/IGMP router. If you only have a single set-top box, I don't think you need it enabled, but if you have more than one, then it allows the upstream server to send only one transmission to your router to serve all of your set-top boxes. So if you had 4 set-top boxes at home, and each set-top box was turned onto the same channel, you could reduce the total bandwidth usage by 75%.

The nice thing about multicast vs. broadcast is that the multicast router knows to send out each multicast transmission only to the devices that have subscribed to it. So your LAN won't be flooded with useless broadcast traffic.

There aren't really any risks to leaving it enabled. But for now you're not likely to get any benefit from it unless your home/office is subscribed to IPTV or some other IP multicast service.

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mDNS anyone? – Ignacio Vazquez-Abrams Apr 12 '12 at 6:38
@Ignacio: mDNS uses IP multicast, but it's purely within your LAN. There'd be no reason to enable multicast from broadband for it. – Lèse majesté Apr 12 '12 at 6:51
You're assuming that the broadband part is connected to the Internet. – Ignacio Vazquez-Abrams Apr 12 '12 at 6:53
@Ignacio: I suppose you're right. That would be another application scenario where you'd want to enable the feature. – Lèse majesté Apr 12 '12 at 7:08
@Lèse majesté - Thanks. Sorry for being a n00b at this but I don't quite follow. From what you mentioned, I understand the basic premise of a multicast broadcast i.e. sending a single transmission to multiple clients who subscribe to it. What I don't quite understand is what role does the router play if the multicast broadcast is say from a host streaming internet TV or radio. So for example, if my router does not have multicast broadcast enabled or doesn't support it, how this affect me listening to internet radio? – PeanutsMonkey Apr 12 '12 at 7:52

I just found a reason to activate that option even for domestic use. Hosts on the wireless and cable interfaces will not see IGMP requests to although they are all assigned to the same network segment.

When that option was activated then Wuala, on Laptop connected through WIFI, found the NAS connected in the gigabit ethernet port. I guess that the complex configuration of briges changes with this configuration.

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Unless the broadband side is a larger network, turn it off. Multicast is used in specific situations to let multiple clients receive a packet without having to explicitly send it to each IP address.

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What do you mean by unless broadband is a large network? What do you mean by `without explicitly sending it to each IP address? – PeanutsMonkey Apr 12 '12 at 7:47
Err... they both mean exactly what I wrote. You're going to have to be more specific about what you don't understand. – Ignacio Vazquez-Abrams Apr 12 '12 at 7:51
The broadband side is always a larger network isn't it and if so, why turn it off? Also my understanding is that although multicast allows multiple clients receive the same packet, it is explicitly sending it to a group of subscribers and not every single IP address on the network. – PeanutsMonkey Apr 12 '12 at 18:59
Well, I'm not counting "the Internet" as a network per se, since it's public. – Ignacio Vazquez-Abrams Apr 12 '12 at 19:00
Thanks. Are you able to give me real world examples of when Multicast is used? – PeanutsMonkey Apr 13 '12 at 0:08

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