Routable IP Multicast addresses

[BVC]

I understand that certain multicast blocks are routable and are not routable (e.g. 224.0.0.0 /24), but I can't find anything on the reserved multicast subnet block (225.0.0.0 - 231.255.255.255 and 235.0.0.0 - 238.255.255.255).

These subnet blocks are listed as 'Reserved' on the RFC website, so from my understanding they shouldn't be really used anywhere. It's just that I've come across an app that uses 225.0.0.0/24 (an internal business app) and I'm struggling with getting it to route across a network (via PIM). I'm guessing this is by design, since it is a 'Reserved' address block?

[marce1000]

 - In general, multicast addresses in the range of 224.0.0.0 to 239.255.255.255 are reserved for use with multicast communication, and they are not routable in the traditional sense. These addresses are used to identify a group of hosts that should receive the same packet, rather than a specific individual host.

The range of multicast addresses from 225.0.0.0 to 231.255.255.255 is reserved for use with administrative scoping, which allows administrators to define multicast groups that are only visible within a particular administrative domain or network segment.

The range of multicast addresses from 235.0.0.0 to 238.255.255.255 is reserved for use with source-specific multicast (SSM), which allows a sender to specify the source of a multicast stream, enabling receivers to filter multicast traffic based on the source.

In both of these cases, multicast packets are typically forwarded by multicast routers using Internet Group Management Protocol (IGMP) or Multicast Listener Discovery (MLD) rather than traditional routing protocols.

 M.

[Harold Ritter]

Hi @marce1000 ,

According to RFC5771 225.0.0.0 - 231.255.255.255 is reserved, but the RFC does not say exactly why it is reserved.

The administratively scoped block is 239.0.0.0 - 239.255.255.255 (239/8)

The SSM block is 232.0.0.0 - 232.255.255.255 (232/8)

The GLOB block is 233.0.0.0 - 233.255.255.255 (233/8)

https://www.rfc-editor.org/rfc/rfc5771.html

You can also refer to the IANA page for more information:

https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml

To answer the original poster it is quite possible that the issue you are having are related to the fact that you are trying to use a reserve block. You should follow the recommendations stated in RFC5771.

Regards,