I've been running FMP13 and FMPAdv13 on Win7 and MacOS10.x platforms, and finally have to upgrade my hardware. I bought a few copies of FMPAdv17 to see how it works, and as far as I can see it will open and run the old FMP13 files, using the same filenames and extensions (fmp12). After a file is created or modified by FMP17, it can still be opened and run normally by FMP13.

It would be great if I could access the files using both FMP13 and FMP17 clients. I don't use Server, just have a machine on my network running a standard FMP13 client. 
Unfortunately, it looks like FMP13 cannot  open any file hosted by FMP17, and visa-versa.
But since the actual file structure seems to be the same (ignoring new functions unsupported in FMP13), I can't see any reason why they can't host each other's files.

Is it flatly impossible?
Or is there something that can be done to one or the other?

Thanks for your time.

Yes it is impossible to open v13 with v17 or vice versa via peer to peer hosting.

FileMaker only maintains 3 versions of compatibility, where as v13 was end of life on Sep 18, 2017

https://www.filemaker.com/support/product-availability.html

Thanks. 
I read the documentation, too, but since they open each other's files fine, I had to ask.