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I have a small lab in my laptop where I have three Virtual Machines. All three virtual machines have Windows 2016 server as the OS. I created a NAT Network for them to "talk" to each other and have access to the internet. All three are using this Nat network and I can ping the other two from one of the machines and they all respond. So, they can communicate.

One of the machine is just a domain controller where I created a "mydomain.local" domain. Nothing else is here.

The second VM I called it "oracle" and it was added to the domain. I can ping it as "oracle.mydomain.local" from any f the other two machines and it responds. In this second VM I installed oracle XE and it works fine from within this "oracle" machine (localhost:1521 works just fine and so does oracle.mydomain.local:1521). The listener of the database is up and running and I can do "telnet oracle.mydomain.local 1521" just fine. So, that tells me there is nothing wrong with it. I also created a rule in the incoming firewall of Windows where I opened port 1521.

In the third machine I have installed a client to connect to my "oracle.mydomain.local:1521" server. I can ping oracle.mydomain.local and it responds. However, when I ping oracle.mydomain.local:1521 I get not response. If I try to connect to the database I cannot. So, it looks to me that the communication is being blocked at some point between the VM's.

So, it looks to me that I am missing something at some point and I think it is between the VM's. However, I cannot pinpoint it. So, please, let me know if I am missing something in my network or my VM's.

Thanks!

Depending on your Oracle DB version ... Oracle RDBMS on windows only uses port 1521 for the initial connect with the
listener and then spawns a new socket to an arbitrary port. So your port forwarding with a NAT network on port 1521
will NOT work.
Solution 1) Use bridged networking an disable the firewall at all.
Solution 2) set environment variable USE_SHARED_SOCKET=true -> all communication will be done by port 1521

OK

thanks for your response. I actually figured it out. It was something in the firewall. I ended up creating a rule where I opened all the ports (it is just a lab with no outside access, so it is ok to have low security) and it worked. But it was definitely strange that even though I had a rule for that specific port it was not working.