I been asked the question if it is possible for an a IP host to use multicast to address 18.104.22.168 in order to reach (and get a response from) another host which is not on the local network segment but on another subnet reachable via a ip router (which do just "regular" routing without firewalling, applying policies etc).
E.g: host 192.168.1.100/24 can reach 192.168.222.100/24 via IP unicast through their common gateway router (which has IP addresses 192.168.1.1/24 and 192.168.222.1/24 on each network).
So given the above example, the question I have been asked is if it is possible to configure the gateway so it is possible for 192.168.1.100 to reach 192.168.222.100 just by sending multicast traffic to 22.214.171.124?
(My apologies, I cannot provide any details about the context where this question emerged other than it is something that is completely isolated from the internet)
Based on my understanding of networking, the answer I am ready to provide is:
"No, it is not possible and if it is made possible somehow, it will break critical networking contracts everyone expects to be kept so don't even try. Your options are:
- redesign the network so both hosts show up on the same network segment
- use unicast between them. Yes, host 192.168.1.100 must be explicitly aware of the IP address of 192.168.222.100 (directly or via name lookups) so you must configure/implement/architect that awareness if it is not in place
- use a dedicated multicast address intended to be routed between subnets. Again, this must be implemented/configured/architected in you solution and yes, you must configure the router accordingly as well
Don't try to use 126.96.36.199 (intended for same-subnet multicast communication) for something it was not designed for in the first place (multicast communication with hosts outside the local subnet)"
Is this response to the 3rd party reasonable? Or have I missed out something fundamental when it comes to multicast or missed some clever technology available in modern routers and servers?
I have quite some networking experience, but multicast is an area where I have little hands on experience, I simply don't know how far you can (or even should) push that specific technology.
Then we have the issue of best practices and what technical, maintenance and economical complexities/pitfalls you want to introduce into a larger picture. Just because you somehow can make traffic directed to 188.8.131.52 reach hosts outside your network does not mean it is a good idea from a development, architectural and support perspective.
(Note, the IPs above are made up in order to protect the innocent, of course...)