Multicast IP Addresses Overview

This topic is to discuss the following lesson:

https://networklessons.com/multicast/multicast-addresses/

Awesome post we’ve got here…

But a couple of things that didn’t quite sync in my mind.

First; When you mention that RIPv2 (224.0.0.9) and basically the range 224.0.0.0/24 is NOT routed between subnets, so my question is how RIPv2-aware routers advertise RIPv2 routes?

Second; Can you give an example of a work with 239.0.0.0/8, cos I fear that if I applied it on a typical LAN (where I’m restricted to use only private IP addresses) that is connected to the Internet, it cause a conflict… Ain’t that right?

Hi M. Bahwal,

Multicast traffic in the 224.0.0.0/24 range is processed by routers but it won’t be “routed” to another subnet. Imagine a couple of routers connected to each other to a switch. They’ll use 224.0.0.9 to communicate with each on this segment. These routers won’t forward these multicast packets to other interfaces.

What kind of multicast example are you looking for? There are many different configurations…

Best Regards,

Rene

“The 224.0.0.0 – 224.0.0.255 range h…”
I think multicast address range is 224.0.0.0-224.255.255.255

Hi Navin,

The 224.0.0.0 - 224.0.0.255 is a “special” range that IANA has assigned for certain applications, these addresses are not routed outside of the subnet. For example, RIPv2, OSPF, EIGRP, etc.

Rene

 

for 224 range : One such example could be “ospf hello” which is not a user traffic but traffic among routers and does not require to be routed.
where-in for 239 range …it is a multicast user traffic . thats how IANA has separated both range for multicast…is that correct to say?

Abhishek,
This is generally correct, with a couple of clarifications:

  1. When you say “224 range” specifically, that is 224.0.0.0/24 which is the range reserved for traffic on the local segment only. Any 224 network higher, say 224.0.1.0/24 has no special significance.

  2. The 239.0.0.0/8 is not really defined as user traffic vs router traffic. Its significance is that it is available for private organizational use. Think of this as being the same thing as 10.0.0.0/8, 172.16.0.0/12 or 192.168.0.0/16 for private IPv4 spaces.

Awesome. clears the doubt now.

Hi Andrew ,
A doubt on higher level …
I was trying to understand connection between 239.0.0.0 AND 224.0.1.0 in MULTICAST concept.
So any video server, once connects to router (so there must be some app that pump video traffic towards router) and router once configured “with respective config” , start sending traffic towards IP 239.1.1.1. And then hosts (typically equipped with an application such as VLC ) that are configured with “related host config” get this feed via that multicast router.

  1. So Is it always 239.1.1.1 or we can change this IP?
  2. where does 224.0.1.0 range comes in to scene ?

thanks.

Abhishek,

  1. The 239.0.0.0/8 range is entirely reserved for private, organization use. This means, the range from 239.0.0.0 - 239.255.255.255 can be used anyway you like–so long as the traffic stays within the bounds of a private address of a company. This means your video server could be configured to use any address between 239.0.0.1 and 239.255.255.254 if your goal to ensure you are using the “private” multicast range.

  2. For 224.0.1.0, I was just trying to make the point that when you originally said “224 range” that in order for your statement to be accurate, you need to make sure you are saying “224.0.0.0/24.” 224.0.1.0 was an example I gave of something that could be considered within the “224 range” but is NOT actually reserved for local segment multicast traffic. There is nothing special at all about 224.0.1.0

What address range can be routable through the internet to make let’s say a public streaming through vlc? Does a streaming service use multicast address? Can multicast addresses pass through NAT firewalls as regular unicast addresses!

Hello Matias

The Internet does not support multicasting. If you attempt to route multicast traffic to the Internet it will simply be dropped. However, you can pass multicast through tunnels (such as a GRE tunnel) over the Internet to allow multicast to take place between two or more sites. However, this does not address your specific issue of trying to deliver a streaming service over the Internet.

Streaming over the internet currently takes place using unicast, since there is no alternative on the internet. Video on Demand services such as Netflix, Disney+, and others wouldn’t use multicast anyway because each user chooses what they want to see individually. Multicast would make more sense if you have a live event that is broadcast simultaneously to multiple users. But as I mentioned before, even then unicast is used because multicast is not supported on the Internet.

Now it is possible to pass multicast through a NAT router using Multicast Dynamic NAT. You can find out more information about this here:

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_nat/configuration/xe-16/nat-xe-16-book/iadnat-multicast-dynamic.html

As far as firewalls go, multicast traffic by default on most Cisco security devices such as ASAs is blocked from any zone to any zone. You must manually create firewall rules that will allow multicast forwarding from a specific zone to a specific zone.

However both NAT and firewall traversal is not that common for multicast since NAT and firewalls are often found at the network edge of an enterprise where it connects to the Internet, and multicast is not routed on the Internet.

I hope this has been helpful!

Laz

Hi,
I see in the above post that multicast is not supported on the internet which makes sense but I just have one question -

What is the purpose of GLOP Multicast ? As its using your Public AS number as part of the multicast address to make the multicast address unique then would there be any way that this could be routed on the internet without being tunneled in gre etc?
or what exactly is the purpose of GLOP multicast when you could just use 239.0.0.0 /8 range or others for your internal network ?
What is the purpose of GLOP ?
Thanks.

Hello Sean

GLOP is an experimental framework that attempted to implement multicast over the Internet. It is defined in RFC 2770 within which it specifically states:

This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested.

So although GLOP was designed for use on the Internet, it was never actually used beyond experimentation. You can find out more info about GLOP at this related NetworkLessons note.

I hope this has been helpful!

Laz

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