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My firewall (ZA) is continually reporting inbound and outbound attempts from IGMP.MCAST.NET.

I have checked on google and found little to identify this. is located in Marina Del Rey, California, United States.

Any idea what this is. Is it XP trying to talk to home?

Mark :?

IGMP is known as Multicast.

This system is used to allow for one stream of data to go to multiple people, rather than the normal method of multiple streams to multiple people.

It is done using a subscription method. Those requests are Multicast subscription requests.

You can safely ignore them.


Many thanks for the reply.

I shall happily ignore it then.

I am not clear about what Multicast is though. When you say 'subscription requests' is this somthing i have subscribed to or am I getting the wrong idea.

Just curious.


You are getting the wrong idea, partly.

It is infact automated subscription. Your computer requests to subscribe, and other systems that need to know get informed as such.

It is the computers way of saying "I'm here and supporting Multicast requests".

More detail on Multicast itself, and what it can do.

The BBC during the Olympics this year, launched a trial, to broadcast high quality video over the internet using Multicast technology. It was only available to Providers that had setup the needed arangment before hand. PlusNet did try to get involved, but not in time for peopel to try it on the Olympic feeds.

The BBC did however provide a lot of live stream regardless of Multicast, but not at the best of quality.

The reason for this, was because of the high costs.

For each person that wanted to watch a tream, they had to broadcast a whole new set of data. The more people that watch, the more they have to transfer. This transfer costs money. Lots of money.

At some point, money runs out. This means that activities must be limited. including things like quality of broadcast, and the total number of people watching.

With multicast technology, they are able to use only a small amount of streams. These feed ISPs like PlusNet, who in turn feed there customers.

It works by your computer sending a subscription request to receive the stream, and later, an unsubscribe request them you hit stop (that should give you a better idea).

In all, because the BBC only has to send out only a few streams, they are able to up the quality of the broadcast.

Hope that explains it a little more.


Firstly, let me apologise for not replying sooner. I have been away.

Thanks for your explaination. Most informative.

Thanks again.