Debian + 2 WinSrv routing how?

Question

Debian

I can't set up the network. Help!
There is Debian, it has a network card with two ports 10.15.20.1 and 10.15.20.3. Mask 255.255.255.0
There are two Windows 10.15.20.2 and 10.15.20.4 connected directly to it. 20.2 to 20.1 and 20.4 to 20.3, respectively.
In this configuration, Windows 1 sees deb, but Windows 2 does not. And deb does not see Windows2.
If I set another subnet on one interface (for example, 10.15.21.3) and on Windows too, then both Windows see deb, do not see each other.
How to make sure that all three of them can see each other?

Answer 1

Take a simple switch and plug everything into it.

Answer 2

Collect two ports in the bridge on Debian, and hang the IP address on the bridge, in Windows at the address from the same subnet. The easiest option.
But keep in mind that this is not a hardware switch, and when running traffic from one Windows to another, you will get some load on the CPU.