subnet mask for Multicast addresses

[oudmaster]

hi guys
I am really confused when I reach to topics
unicast, multicast and broadcast with subnet mask in IPv4.

so does multicast have a subnet mask ?

 

[ShadeTreeTech]

Two different things. A subnet mask just tells your computer what IP addresses are local and what aren't. (Why would you use your cell phone to call your friend, if he is standing in the room next to you? You wouldn't.)

Multicast refers to the fact that you are sending data to multiple IPs rather than a single IP. (How many people are you telling at the same time?) You might end up using your cell phone (router), to contact those additional people so you can say what you need/want to say only 1 time.

I hope this sheds a little light on the subject.

 

[bill001g]

Most the confusion is multicast only appears to use IP addresses. These ip are generally referred to as multicast groups. They should be more thought of as an identifier rather than some unique address as we think of with IP. For example you can have 2 machine at completely different locations transmitting to the same multicast group address and it will all work fine as long as they are using different ports. Subnet masks when used in conjunction with multicast are used to limit which multicast groups a location can use. It just limits which groups a router will accept requests to join a feed or transmit data to. It is not so much a technical limitation but a way to implement corporate policy for example that certain sites take the multicast groups out of certain ranges. It may be used to prevent certain type of traffic from going from site to site but there are other ways to do this.

The key to remember is multicast is only sent to machine that request it. Even though it appears to work like broadcast it does not because the switches are smart and know exactly who wants which multicast feed. The newest versions of IGMP can even detect the port so if your were to have 2 feeds to the same multicast IP the switch would filter it so you only get the single port you want. In addition the PIM protocols used to transmit multicast between sites (ie routers) also know if a user at a site needs the feed and will not send it until there is a request.